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finds the ancient crystalline rocks exposed in Lukenya Hill, and in the valleys of the Kitengela and Mbagathi rivers. In the former case the strike of the foliation planes is N. 20° E., while in the latter exposures the strike is practically true north.

Directly overlying the crystalline rocks the first group of volcanic rocks is exposed. As the observer proceeds in a westward and subsequently in a north-westward direction over the gently rising ground to the boundary with the fractured zone, he encounters in succession the next three groups, each resting on the uppermost component of the group next below in the order of deposition. The easterly limits of the groups are often marked by relatively steep slopes, forming the eroded terminal walls of the more resistant lava sheets composing them, which have flowed from the west or north-west. It is found, in fact, from the stratigraphical sequence as revealed in the valleys, that the major centres of dispersion of the volcanic rocks must have been situated to the westward within the fractured zone, and that the younger groups have a lesser distribution within the undisturbed zone than the older ones, pointing either to a decrease of vulcanism, or the shifting of the main centres of eruption to the westward, or both combined. Although the major vents were situated within the fractured zone (except some of those of the fourth group), a number of subsidiary vents arose within the undisturbed zone on the eastern flanks of the trachytic (Group III.) volcanoes, for there occur at intervals, associated with the major sheets of that group, short streams of a rapidly cooled lava, which was originally glassy, but has now become devitrified by chemical alteration, in contiguity with the products of explosive eruptions.

Individual lava sheets are as a rule attenuated; it is exceptional for a sheet to have a greater thickness than 100 feet. The sheets of lava of any particular group may follow one another directly or have tuff intercalated between them. The proportion of tuff to lava in different groups has not been evaluated. In the volcanic rocks of the first group and, to a much lesser degree, in those of the fourth group, lava is much in excess, and must have been highly liquid on extrusion, spreading fanlike from the centres of eruption over wide areas at flat gradients-a type of vulcanicity more usually associated with flows of basic than of intermediate lavas. A typical example, in the Nairobi District, of this type of vulcanism is the fan-shaped sheet of lava (the upper sub-group of Group I.) which composes the Athi Plains. The exposed portion of this sheet covers an area of 126 square miles, and varies from 100 to 150 feet or more in thickness; it has an average surface gradient in the direction of flow of 1 in 480, which is probably not greatly different from the gradient at which it solidified. The type of vulcanicity which gave rise to the rocks of the remaining groups was much more explosive; for the proportion of tuff and breccia to lava is much higher, and in some cases probably greatly exceeds 50 per cent.

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Fan-shaped arrangement of columns in Group IV. lava of Dome, south of Nadu ridge

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One of the wedge-shaped terminations of the N.-S. ridges of the grid in Group IV. rocks, where cut by the Kedong scarp bearing S.S.W.

No palæontological evidence, adequate to enable the age of the volcanic groups to be determined, has so far been forthcoming from the Nairobi District. Lacustrine sediments, sub-aqueous tuff, river alluvia and soils, interbedded between components of the series, are occasionally exposed and have yielded plant remains, but none of those so far procured has proved to be sufficiently limited in time of existence or determinable specifically to permit of the ages of the deposits to be fixed with certainty. Relying on correlative evidence, Professor Gregory ('The Rift Valleys and Geology of East Africa,' pp. 105 and 193-209) has adduced a tentative classification by period of the volcanic rocks of Kenya. According to that classification, the groups of volcanic rocks in the Nairobi District would probably be placed as follows :

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It is to be hoped that future research in the sediments will yield certain evidence of the age of the groups. It is undoubtedly the case that the oldest volcanic rocks (Kapitian) have suffered greatly from denudation, and that there were varying, and in some cases considerable, lapses of time intervening between the other groups.

Adverting now to the structure of the fractured zone-or eastern flank of the Rift Valley in the Nairobi District-we find topographical features so unusual and so different from those which characterize the undisturbed zone that they would excite the curiosity of the most casual observer. The structure of the fractured zone has many points in common with the adjacent region to the southward, described by Dr. Parkinson in "The East African Trough in the Neighbourhood of the Soda Lakes" (Geogr. Fourn., 44, July 1914).

In place of the valleys carved out by erosion in a west-to-east direction along the maximum gradients of an inclined plateau, which carry the surface drainage of the undisturbed zone, there is developed a series of valleys trending in a general north-to-south direction and bounded occasionally by steep cliffs on both sides, but more usually by a steep cliff on one side and a comparatively gentle slope on the other. A type of topography has been produced which is characterized by parallel ridges and valleys following one another in close succession, and which has been called "grid-structure" and "fault-block structure." A traveller from east to west in this district across the trough, in one of its most jagged parts, is impressed by the apparently interminable number of ridges which he has to climb-in section like the teeth of a saw on a gigantic scale--for he has no sooner ascended one cliff and descended a boulder-strewn slope into the ensuing valley, than he is faced with the ascent of the next cliff. Photographs fail to convey an adequate impression of the succession of long narrow gorges; and sections and maps only enable a partial visualization to be achieved. It is only by ascending to a commanding position, such as the summit of Ol Donyo Nyuki or Najele, when the rising or setting sun illuminates, and causes to stand out in relief, the cliffs of the cañons to the southward, that a full realization of the nature and unusual character of the topography of the valley is conveyed to the mind.

An inspection of the maps and sections will show that in the Nairobi District the rocks exposed in the fractured zone are entirely volcanic, and there is no direct evidence from the district of the level of the underlying ancient crystalline rocks. The highest and lowest points above sea-level in a section from east to west across the eastern flank of the valley are 8070 feet at the summit of Ngong Hill, and 3550 feet at the southern exit from the Enkobirri Gorge that is a maximum difference of elevation of about 4500 feet in 13 miles, or an average maximum gradient of 6 per cent. from east to west. Other parts of the flank in the Nairobi District have gradients between that figure and 5 per cent. The average slope of the bottom of the valley approximately from north to south, along the Uaso Kedong, Barajai, and Enkobirri Gorges, is about 11⁄2 per cent. It will be seen that the eastern flank of the Rift Valley in this district-and indeed the western flank also, as shown by Section FF -is of a very different character from that of most of the major troughs of the Rift Valley System, where vulcanism either has not manifested itself, or has been of minor importance; for one finds the flank composed of a "grid of small parallel ridges and valleys, which have been designated "miniature rift valleys," in place of flat-floored depressions, confined within two boundary walls, as in the case of the Nyasa, Tanganyika, Albert and Edward depressions.

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As the eastern flank is traversed from east to west in the descent to lower levels, the climate is found to become drier; the well-watered, soil-covered slopes of the undisturbed zone, with their grass lands, forests, and cultivation, give place to cliffs and boulder-strewn slopes, scantily clothed with thorn-scrub and isolated grass tufts, in a semiarid climate. The valleys contain no permanently flowing water, and only one permanent waterhole, namely Enkobirri, exists in a distance of 40 miles between the springs of Ngong Hill on the eastern side and the Uaso Nyiro River on the western side of the valley. Humid air, drifting from the eastward with the prevailing winds, is cooled as it ascends the inclined plateau of the undisturbed zone, parting with most of its moisture; it then encounters the heated air rising from the valley and the rain-clouds become absorbed. Much of the rainfall within the valley is discharged during thunderstorms. Although considerable local precipitation then takes place, it is rapidly removed from the surface by soakage into the

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