may have been the base of the sial, while the subsequent eruptions appear to have been differentiation products of the portion of original magma, which had moved upward. Moreover, the silica percentage of this lava differs by only 4 per cent. from the average of the Atlantic and Pacific magma-basalts analyzed by H. S. Washington (Bull. Geol. Soc. America). The initial eruptions produced sheets of lava at gentle gradient, while the subsequent eruptions were generally associated with cones of tuff and the lavas solidified at steeper gradients. Dr. Evans has advanced a theory (Nature, 8 August 1925, p. 212) to explain the origin of soda-rich magmas by the pre-crystallization from the sima of high-pressure minerals rich in magnesia, iron, and lime, resulting in the removal of a large proportion of those constituents from the magma so affected, and consequent richness in soda of the remainder. The not infrequent occurrence of garnet-gneiss in the ancient crystalline rocks of Kenya Colony, which may be altered intrusions in the very old group of metamorphosed sediments, named by Dr. Parkinson the Turoka Series (Quart. Fourn. Geol. Soc., 69, pp. 534-539), may not be without significance in this connection. These ancient sediments apparently have a wide distribution in Kenya Colony (Glenday and Parkinson, Abst. Proc. Geol. Soc., 6 January 1926). The sequence of events in the region of the trough in the Nairobi District during most of Kainozoic time points to the existence of eastwest tension which varied periodically in intensity. As pointed out by Sollas, and often reiterated by others, normal faulting in general results from the action of tensile forces. Difficult and controversial though the subject of vulcanism is, the conclusion that the volcanic activity as well as the gravity faulting resulted from crustal tension, however induced, seems unavoidable. Doubtless other causative factors contributed their quota, but it seems inconceivable that periodically recurring vulcanism followed by subsidence along dominant joint planes, having a nearly constant direction, could have occurred while the upper part of the crust in the region of the earth-movements was either under horizontal compression or free from applied force normal to the planes of subsidence. When the intensity of tensile stress became great enough, yield would express itself by liquefaction of rock at depth and extrusion at the surface, on account of expansion on fusion and release of gases. As cooling progressed after relative stability had been attained, the accumulation of mass above and deficiency at depth would cause adjustment by superficial subsidence. Three main groups of meridional tension faulting, following successively the three periods of major vulcanicity, are recognized by Professor Gregory, and the age of these movements he regards as Oligocene (preNyasan), Pliocene (Naivasha), and Pleistocene. The principle that transverse movements in conjunction with rejuvenation of the old meridional fracture planes caused the basins of the Rift Valley in Kenya is also enunciated by him ('The Rift Valleys and Geology of East Africa, 1921, p. 216). The evidence from the Nairobi District points to four or possibly five major movements having followed periods of vulcanicity, but these movements were comparatively local and largely confined to the rocks produced by the period of vulcanicity, which they followed. Investigation has failed to yield any evidence in the district of transverse folding amongst the volcanic rocks. Regional pressure in all directions seems to have been absent throughout. The not infrequent gentle reversals of dip in sheets of lava, often accompanied by slip faulting, which are clearly shown on north-south cliff faces, are regarded by the writer as having been caused partly by selfadjusting movements in the sheets of volcanic material as they cooled, contracted, and sank unequally, and partly by deposition over oppositely sloping surfaces. The movements along the major cross faults also show no evidence of north-south compression; on the contrary, they point to its absence. Most investigators have regarded the East African troughs as having been caused by tensile stress, but a few have urged the greater probability of their origin as structural features by compression, notably Mr. Wayland, Director of Geological Survey, Uganda (Geogr. Fourn., November 1921), as a result of his investigation of the Albertine trough between Uganda and the Congo. Mr. Wayland also concludes ('Petroleum in Uganda,' Appendix A, 1925) that compression caused movements in the Lake Albert trough from the Oligocene to the Middle Pleistocene. If this conclusion is correct it would appear that the Eastern Rift Valley was subject to tension and the Western Rift Valley to compression during Kainozoic time. It is to be observed, also, that Krenkel (' Die Bruchzonen Ostafrikas, 1922) and other German writers have found the faults of the troughs in Tanganyika Territory to be normal ones, and therefore tensional, while the results of the gravity survey of Kohlschütter conform with this view. It is conceivable that the formation of the Rift Valleys as structural features antedated the first eruptions; in fact, where unrejuvenated flanks of the valley in Kenya are composed of ancient metamorphic rocks, they are generally gentle slopes more deeply eroded even than the earliest lavas. No evidence pointing to its original development as a structural feature by compression causing reversed faulting and overthrusting has, however, so far been adduced in the case of the Kenya trough. A study of the physiography of a very small section of the Rift Valley system without correlation with that of other parts is necessarily inadequate to justify any attempt being made in this paper to examine the theories which have been advanced to account for the stresses which gave rise to the East African Rift Valleys. The theory advanced by Professor Gregory, based on the doming of Eastern Africa and the subsidence of the Indian Ocean, with effect from the Cretaceous to the Oligocene and the wrench caused by the Eurasian mountain-building movements between the Oligocene and the Pliocene (Geogr. Fourn., 1920) is well known. A new light has been thrown on the difficult subject of crustal tension by Professor Joly ('The Surface History of the Earth, 1925, and two earlier publications). Much additional data, together with mathematical treatment on the lines adopted by Dr. Jeffreys ('The Earth, 1924), seem necessary before any individual theory of the origin of Rift Valleys commands general acceptance. The writer considers that the evidence so far noted in the Nairobi District, and adjacent districts, leads to the following summarized conclusions with regard to the Nairobi District : (1) The earliest eruptions were of the "massive" type and representative of the original magma. Subsequent eruptions were products of differentiation of the original magma and were largely of the explosive type. The major centres of eruption were situated within the fractured zone. (2) The volcanic rocks are divisible into six groups, each marking a period of volcanic activity and followed by a period of comparative quiescence. (3) From the earliest manifestations of vulcanism, the Rift Valley has been in a state of regional east-west tension without conjugate north-south compression. (4) Following each period of vulcanism, as represented by the corresponding group, gravity or slip faulting took place along many closely spaced planes of weakness directed approximately meridionally, giving rise to the "grid" or "fault block" structure. (5) Meridional direction was given to the succession of closely spaced planes of subsidence by (a) the dominating series of contraction joints being developed in each group parallel to the scarps of pre-existing grids; and (b) maximum vertical contraction of lavas taking place in and over the pre-existing valleys of those earlier grids. These factors initiated the parallel lines of weakness along which subsequent gravity faulting took place, by reason of the continuous east-west tension. (6) The faulting following the extrusion of a group was chiefly confined to the material erupted during that period of vulcanism, the older groups having acquired relative stability. The throw of the faults was greatest where the mass extruded was greatest. (7) The major centres of activity of each group within the fractured zone subsided along cross faults, and the pre-existing meridional main faults, which were thereby rejuvenated. The result was the production of basins in place of highlands formed of the greatest mass of material. The cause of subsidence was the accumulation of material over the surface and consequent deficiency at depth in the area where the major centres of eruption existed, in a region of continuously operating tension. These basins were subsequently overwhelmed by later products of eruption on recrudescence of vulcanism. (8) The present form of the "miniature rift valleys" of the grid is largely due to erosion of the valleys which had been initiated and partly developed by jointing and faulting. THE GROUPING of forms IN NATURAL SCENERY Vaughan Cornish, D.Sc. Read in abstract at the Oxford Meeting of the British Association (1926) in Section E (Geography). I. H ARMONY and Disharmony in Nature.-Men of science have studied the structural forms of Natural Scenery in detail but have given relatively little attention to their apparent grouping, upon which the beauty of Landscape largely depends. Neither can we learn the condition of harmonious grouping in Natural Landscape from the landscape painters, for even those who believe in being "true to Nature," in the sense of following natural forms, do not "copy Nature" in the sense of accepting the grouping of forms which confronts them in the landscape. The reason for re-arrangement, or composition," has not only to do with the material restrictions of painting but with the fundamental fact that disharmonies of form occur in purely Natural Scenery, and that it is only when and where there happens to be a broad angle of outlook free from an intrusive disharmony that the eye finds perfect satisfaction in the landscape. The opinion of many Nature-lovers that the appearance of purely natural surroundings is always harmonious is largely due to the cooperation of the senses. Fresh air and fragrance, the song of birds and the rustle of the leaves, crisp cold, balmy warmth, and the salt sea breeze all play their part and help to make us more than satisfied with what we see. To point out visual disharmonies to the contented Nature-lover seems an ungrateful task, but has its use, for there are beautiful relations of earth and sky, of land and sea, of tree and mountain, there are broad effects and subtle effects, which will never be fully appreciated unless the conditions of visual harmony are critically studied. 2. Land and Cloud.-Tradition apart, there are conditions when the heaven actually appears as a vault or dome. A hill overlooking the lowlands of Southern England with a broadly stretching sky-line of treeless downs 20 miles away sometimes provides such a picture when detached clouds dot the sky. The beauty of the scene is more than the The forms of the land are comprehensively described in The Scientific Study of Scenery,' by Dr. J. E. Marr. beauty of its two parts. The satisfaction which it gives is not merely due to the pleasure of a panorama of the plain and the pleasure of an uninterrupted view of the clouds, but to the harmonious combination of land and sky. I made sure of this conclusion when out on Old Dean Common near Camberley, with a broad view westwards to the distant downs of Hampshire and Berkshire on a summer day of blue sky and drifting white cloud. The top of the common is a level plateau covered with short heath, and my standpoint was 100 yards back from its horizontal edge. Reclining on my side, I adjusted the height of the eye so that the edge of the plateau just covered the distant downs. Although no part of the sky was cut off, the vault was no longer vast in appearance. On standing again erect, the vault, seen as the roof of a wide instead of a narrow world, regained its grandeur. I noticed that the detached white clouds overhead were not drawn down by the extension of the rim of sky as might have been expected, the camber of the arch remaining bold. The impression of the vault is enhanced by the completion of the panorama from an isolated eminence, such as the Cairn on the hill above Llandrindod, with its wide view of Central Wales. I find that from this point the eye can take in only one-sixth part of the encirclement of hills at once, from which it may be safely inferred that the full effect of the vaulted roof is obtained by a mental synthesis of separate views. Cumulus, the cloud of pre-eminent form, rises from a flat base, and at any given time the bases are at about the same altitude. We know these clouds to be the white caps of great billows of air, but, beautiful as this scientific conception is, the natural man within us delights most in those views in which the pile of clustered domes does not appear suspended, but rises from misty shadow so as to produce the perfect illusion of a range of lofty mountains. Towards the close of a fine summer day I noticed a range of cumulus above the northern horizon which, except for the difference between domes and pyramids, reminded me of the Bernese Alps as seen from Oberhofen, on the lake of Thun (where the Mönch stands up more than 6°), and I found on measurement that the summits of the clouds were 7° above the level of the eye. Cumulus in numerous detached portions has a remarkable perspective grouping towards the horizon. This grouping shows best at sea, where the strong level sky-line imparts unity to the clouds, whose level bases are narrowed by perspective. This picture is seen to perfection near the northern edge of the Trade Winds on the voyage to the West Indies, where the sea horizon is frequently very clear. Trees, which often combine effectively with clear sky, spoil the view of cumulus, for they neither allow the perspective arrangement to be well seen nor make a pattern with bright masses which show through the perforated screen of foliage. Individual cumulus clouds standing somewhat high in the sky, and therefore fairly near, often subtend so large an angle that they will only group with a landscape free from |