PRE-CAPE ROCKS OF THE NORTH OF THE COLONY 95 



Though the blue mineral was described under the 

 name of crocidolite, and is so called in all the text-books 

 of mineralogy, that term is commonly used in this 

 country to denote the various altered forms due to the 

 oxidation of the ferrous iron and the replacement of the 

 greater part or even the whole of the iron and all the 

 soda, magnesia and lime by silica. The extreme pro- 

 duct is white fibrous quartz, but all the intermediate 

 stages between this and the unaltered blue crocidolite 

 can be found. The variously tinted brown and yellow 

 mineral, which takes a high polish and which is made 

 into ornaments on account of the beautiful shades of 

 light reflected from its fibres, is in one of these stages 

 of alteration. Crocidolite or its altered form is found in 

 the Lower Griqua Town beds throughout their extent 

 in the Colony, from Prieska in the south to beyond 

 Heuning Vlei in the north. It seems to be more 

 abundant in the lower half of the group than the upper, 

 but it is not confined to the former. Even where it is 

 not present in sufficient quantity to give the rock a 

 characteristic appearance, it is often found in examining 

 thin sections under the microscope. 



Quartzites, sandstones and grits are occasionally 

 found at various horizons, in the Lower Griqua Town 

 beds, but there is no one band of such rocks which ex- 

 tends over a very wide area. 



Limestones are occasionally found in the lower part 

 of the group, but their chief position is near the top, 

 usually about 100 or 200 feet below the Ongeluk lavas. 

 In appearance they are like the more ferruginous 

 varieties of the Campbell Kand limestone. They vary 



