liev. S, 8. Donuin — Geology of Bm^itoland. 115 



fragments about 10 or 12 inches in length, not unlike spear-heacls. 

 At another place, a little higher up, they are ropy and scoriaceous. 

 The whole crest of the mountain consists of doleritic amygdaloid lavas, 

 interbedded with vesicular varieties and very thick beds of ash. The 

 vesicles in the lavas are generally filled with calcite. An exposure 

 of the junction of the lavas and the Cave Sandstone occurs in the 

 bridle-path upon this mountain, and indicates that erosion of the Cave 

 Sandstone had taken place before the deposition of the Volcanic Beds, 

 but the evidence is not decisive. The erosion may be due to a 

 subsequent and different cause. This does not apply to an exposure 

 near Sefikeng, which plainly indicates erosion. Many of the highest 

 summits, such as Mount Hamilton, Mont aux Sources, Bitsolebe, etc., 

 contain a vertical thickness of over 4,000 feet of lavas and ashes. 

 These mountains, with the exception of the last, are situated in very 

 difficult and broken country, far from centres of population, and as I 

 have not visited them personally I am unable to describe them in 

 detail. 



Connected with the lava beds are a great series of intrusive dykes 

 and sheets. They cover a considerable extent of country, and traverse 

 all the Stormberg Series. They are subsequent to the Volcanic Beds, 

 and so far as I can make out are of the same age as the Karroo 

 dolerites of the Cape Colony and of similar composition. Amongst 

 the many scores of them that I have examined I cannot say that 

 I have ever observed any decided characteristics of a lava-flow. 

 Doleritic lava can be generally separated from doleritic intrusions, not 

 only by its appearance but also by its mode of occurrence. A doleritic 

 sheet weathers red, which doleritic lavas never do, at least so far 

 as I have seen them. Some of the dykes are only a few inches wide, 

 and upwards of 50 feet long, sending out side branches. By far the 

 larger number are from 10 to 20 feet in thickness and several miles in 

 length. They traverse the country in all directions like great roads 

 running up and down the hills, and usually terminating in a large 

 sheet. There are many examples of this kind, as for example at the 

 mountain Thaba Tsuen. It crops out in the valley to the west of the 

 cone ; sends two parallel dykes up the mountain at very steep angles, 

 and separated by rather less than a mile. These dykes traverse the 

 lower lava beds, reappear on top as two shallow parallel trenches, and 

 then pass into an immense sheet nine or ten miles in length by four in 

 width, which attains in places a thickness of 400 feet. Near Mohale's 

 Hoek two remarkable examples of intrusive dykes are to be seen. 

 The first runs across the country with perpendicular sides like a huge 

 wall for about seven miles, traversing the lavas of the Mokhele range. 

 At one point where a stream has cut through the side of the dyke the 

 dolerite assumes a columnar structure closely resembling basalt. The 

 Molteno sandstones through which it has broken are much altered. 

 The other dyke is a conspicuous feature of the landscape, and runs in 

 an east and west direction for upwards of 15 miles from the volcano 

 Mokhele to Eaboroko. It is perfectly straight, 30 feet thick, and 

 about the same in height, but more must be hidden by superficial 

 accumulations. It stands up in the form of a huge causeway, and is 

 much weathered at the top and sides, the nodules ranging in size from 



