41^8 EEV. a. BLENCOWE ON CERTAIN GEOLOGICAL FACTS 



rises from- its foot to more than 1500 feet. But just at its present 

 foot a mass of basaltic trap is found resting on the shale, and 

 bounded on all sides but its perpendicular face by the sandstone 

 and shale of the TJmsinga and an adjoining denuded ridge. The 

 distance from the top of this isolated mass of trap to the base of the 

 trap capping of the Umsinga is not less than 1000 feet, and the 

 horizontal distance is not more than a third of a mile. The sur- 

 rounding strata, so far as seen, are undisturbed. 



The peculiar feature of the geology of the Orange Eree State at its 

 line of junction with N^atal is the existence of isolated mountains of 

 sandstone and trap, which rise to an average height of 2000 feet from 

 the present general level, and which, from their sameness of structure, 

 evidently at one time formed parts of a continuous line of country 

 of the same elevation. Through the whole of this line there is no 

 evidence of volcanic eruption by which the trap has been thrown up, 

 nor is there present indication of the abrading force by which the 

 intervening mass of trap and sandstone has been carried away. 



In the south-western part of the Transvaal, near Potchefstroom^ 

 occur rounded hills of a creamy white stone, with small red specks 

 like oxide of iron : it is of very fine granulation, dense and opaque. 

 It is not lime, and cannot be described as quartzose. 



In the central portion of the Transvaal, the Magaliesberg runs 

 east and west about 70 miles. This range is a series of sandstone 

 upheavals presenting a face from 400 to 700 feet deep, with a back 

 slope of several miles. In some cases the sandstone in this range 

 is in a quartzose condition. 



About 30 miles to the north-east of the Magaliesberg we enter on 

 the large metallic deposits which are the distinguishing mark of this 

 part of the Transvaal. Mountains of very rich iron-ore succeed 

 each other up to the Gold-fields, near to which hsematite abounds 

 in horizontal slabs and nearly perpendicular ridges. 



Pilgrim's Eest, the most extensive and richest Gold-field yet 

 wrought, is a gorge running at right angles to the Blyde river, in 

 an easterly direction, at an average depth of 1000 feet. The tops 

 of its un denuded sides are covered with lava ; all the gold found in 

 it is molten ; and calcined quartz to a depth of 18 inches covers the 

 original bottom and sides of the excavation, except in the course of 

 a stream which at some former period has poured with great violence 

 through the gorge, so as to round the blocks of lava with which its 

 bed was covered. 



Immediately over the ridge, at the eastern end of Pilgrim's Kest, 

 the Mac-Mac diggings are found, where the violent aqueous and 

 igneous actions just referred to have not prevailed. There is not the 

 capping of lava to the hiUs ; and the gold found here is bright and 

 angular, as though it had just fallen from its quartz matrix. 



