114 Rev. S. S. Dornan — Geology of Basntoland. 



comb. Most of the lavas are basalts, but andesites occur, not only 

 intermingled with the others, biit composino- entire hills themselves, 

 as at the magistracy' of Mayeni in South Basutoland. Agglomerate 

 necks are of frequent occurrence, filling up the great vents, and also in 

 small isolated necks. Most of the indications presented by the lavas 

 of Basutoland point to their deposition in water. This, however, is 

 difficult to reconcile Avith the fact that in two instances where the 

 junction of the lavas and the Cave Sandstone is exposed, the former 

 rest on the eroded surface of the latter. The upper members of the 

 Cave Sandstone seem also to have been removed by denudation prior to 

 the deposition of the volcanic beds. In the lower flows of the group 

 thin intercalated beds of sandstone occur, pointing to an interruption 

 of volcanic activity, the thickest being 3^ feet. A short description 

 of two of these volcanic peaks, Thaba 'Telle and Thaba di Noha, will 

 serve as examples of all the rest. 



Thaba 'Telle is a moxmtain about 7,800 feet high, rising abruptlj^ 

 from a platform of Cave Sandstone. Its lower slopes consist of 

 doleritic lavas, alternating with vesicular varieties and beds of purple 

 ash, and at the top of agglomerate, evidently the remains of the old 

 throat. The lavas are just under 2,000 feet in thickness. The 

 mountain is a kind of three-sided cone, with steep, often precipitous 

 sides, and was once much more extensive than now. The summit is a 

 plug of naked agglomerate standing up to a height of fully 100 feet, 

 exceedingly steep and in some places overhanging. It can only be 

 climbed at one point where there is a crack left by the weathering 

 away of a doleritic dyke, and then only with considerable difficulty 

 and risk. The top is convex and grass-grown about 30 square yards 

 in area. The view from the summit is magnificent, as several 

 prominent volcanic peaks can be seen in the distance. This plug is 

 the resort of multitudes of vultures that make their homes in the 

 cracks and fissures of the rock. The lavas of Thaba 'Telle are full of 

 steam holes, more especially in the upper beds, and about the middle 

 of the beds are two thick deposits of ash separated by a thin bed of 

 amygdaloid lava. This alternation of beds can be made out at some 

 distance, as the ash is light jiurple in colour, and the lava a deep 

 shining black. The same phenomenon occurs elsewhere. Near the 

 base of the mountain is a large intrusive sheet surrounding what was 

 once a subsidiary cone, but what is now nothing more than a mere 

 plug of doleritic lava and agglomerate. This sheet is of much later 

 date than the surrounding lavas which it penetrates and overlies. 

 From it spring two immense dykes that traverse the country for miles 

 in practically straight lines. 



Thaba di Noha. This mountain is about 50 miles, as the crow flies, 

 from Thaba 'Telle, and rises immediately behind the magistracy of 

 Mohale's Hoek. It is the extreme western point of the great Mokhele 

 range. It is about 7,500 feet high. The bridle-path from Mohale's 

 Hoek to IVIoyeni leads over the plateau from which the mountain rises, 

 between the plug and what is presvimably the old crater wall. It is a 

 typical volcanic vent, and one can trace the outflow of the lavas on the 

 north side of the hill. At one point where a stream has cut through, 

 they are steeply inclined, glassy, and weather into coarse splintery 



