Notices of Memoirs—W. Gibson—Geology of Hast Africa. 561 
is the occurrence of dykes, which form prominent walls, and may be 
traced for long distances across the country. 
The greater number of them converge towards a ieentra in 
the highest ridge in the middle of the drainage basin of Crandall 
Creek. A small number converge towards a second centre three or 
four miles east of the first. In the southern part of the district 
there are many dykes trending towards a centre near the head of 
Sunlight Basin, about fifteen miles south of the Crandall centre. 
‘The centre towards which the Crandall dykes converge is a large 
body of granular gabbro graduating into diorite. It is about a mile 
wide, and consists of numerous intrusions penetrating one another, 
and extending out into the surrounding breccia, which is highly 
indurated and metamorphosed in the immediate vicinity of the core. 
Within the area of indurated breccia the dyke-rocks become rapidly 
coarser-grained as they approach the gabbro core. This was un- 
doubtedly the central conduit of an ancient volcano, the upper 
portion of which has been eroded away. 
Upon comparing the geological structure of this region with that 
of an active volcano like Htna it is apparent that the lava-flows 
which form the summits of the outlying peaks must have been 
derived from lateral cones fed by dykes radiating from the central 
conduit; and, assuming that the volcano of Crandall Basin was 
similar in type to that of Htna, an idea of its original proportions is 
derived by constructing, upon profile sections through the Crandall 
core, the outline of Etna. If the erosion of the summits of the 
highest peaks is neglected the resulting height of the ancient volcano. 
above the limestone floor is estimated at 13,400 feet. This is un- 
doubtedly too low, and is well within the limits of present active 
volcanoes. 
Hrosion has removed at least 10,000 feet from the summit of the 
mountain to the top of the high central ridge in which the granular 
core is situated, and has cut 4,000 feet deeper into the valleys on 
either side. It has prepared for study a dissected voleano, which, it 
is hoped, will in time reveal some of the obscurer relationships 
existing between various phases of igneous rocks. 
XIII.—Geroxocicat Sxercn or Canrran Hast Arrtca. By Watcor 
Gizson, F'.G.S. 
{\HE tract of country described in this paper is situated in 
Equatorial Hast Africa. It extends from the coast inland 
to the N.W. borders of Victoria Nyanza. 
The small island of Mombasa, the starting-point of the expedition, 
lies fitfy miles north of the island of Pemba. A narrow creek, 
fordable at low water, separates the island from the mainland. 
The sea cliffs are composed of coral rock, which also forms an 
inland belt about two miles broad, with a general elevation of 50 
feet, which sometimes rises to 100 feet. A fringing reef borders the 
coast. The shore sand consists of comminuted corals and shells 
mixed with rounded fragments of quartz, orthoclase, garnets, and 
splinters of clear blue cyanite. These constituents appear to be 
DECADE III.—VOL. X.—NO. XII. 36 
