﻿250 



DK. A. HOLMES ON THE TEBTIA-BI 



[vol. lxxii,. 



XV. Basalts: Lava-Flows. (PL XXI, fig. 8.) 

 The specimens collected include the following : — 



124. Brown amygdaloidal basalt, near the bend of the Monapo River, 

 — south side. 



125. Red amygdaloidal basalt, between Mochelia and Mitikiti (penetrated 



by an andesite dyke). 



131. Purple vesicular amygdaloidal basalt; half a mile south of Sokoto 



Hill. 



132. Purple vesicular basalt ; three-quarters of a mile west of Sokoto Hill.. 

 133. 1 



134. I- Brown amygdaloidal basalt ; south-east of Ibrahimo. 



135. J 



136. Purple vesicular amygdaloidal basalt ; east of Murimatigri. 



145. Purple-brown amygdaloidal basalt ; near the Sanhuti River. 



146. Grey-black vesicular basalt ; near the Sanhuti River. 



One of the best and most continuous exposures of amygdaloidal 

 and vesicular basalts is to be found between the Monapo and 

 Mitikiti Rivers. Numerous small streams and long alluvial Hats, 

 connected with the sea by way of mangrove swamps, cut up the 

 flows, and sometimes expose the underlying floor of shale or lime- 

 stone. In places, hard dykes stand out in ridges running nearly 

 due north and south. The basalt is a rough purplish-grey rock, 

 sometimes dull and earthy at the surface, but often glazed b\ the 

 deposition of a thin film of lateritic constituents. Along the 

 military road (made in 1910) which skirts the northern shore 

 of Mokambo Bay, the freshly-broken rocks could be admirably 

 seen in situ. The colour of the unweathered rock is there 

 purple or reddish-brown, variegated by the green linings of the 

 amygdales and brightened by the sparkling cleavage-surfaces of 

 innumerable crystals of zeolites, such as heulandite. Similar 

 exposures were examined east of the forts at Murimatigri and 

 Ibrahimo, and in the Sokoto-Hill district. The thickness of the 

 flows is now very variable and much reduced by erosion, but it was 

 nowhere seen to, exceed about 40 feet. 



The lavas are characterized by a high proportion of glass — dark 

 red-brown to black — and by a strong development of vesicles 

 and amygdales. The chief minerals present in the latter are 

 chlorite, zeolites, and various forms of silica. It is noteworthy 

 that calcite is extremely rare. Chalcedon}^ is very abundant, and 

 occurs in a great variety of forms including waxy earnelian, red 

 and green jaspers and bloodstones, and beautifully banded agates. 

 The interior of many of the siliceous amygdales is occupied by a 

 mosaic of quartz. In other cases, where a hollow cavity has been 

 left, well-formed crystals of quartz, or zeolites, project from the 

 sides. Among the agates and jaspers found in the alluvium 

 Hanking the Monapo, Mr. Wayland discovered a number of worked 

 stones and implements. These are figured and described in an 

 interesting paper, published in 'Man,' July 1915, p. 57. 



Under the microscope the minerals seen are identical with those 

 of the dyke-rocks already described. The felspars in some 



