THE KARROO SYSTEM 169 



others of volcanic origin. The name " Dwyka Conglo- 

 merate " was introduced by Dunn from the occurrence 

 of the formation on the Dwyka Eiver near Prince Albert, 

 and this term has been almost universally adopted. 



The rock is typically blue or greenish in colour, com- 

 pact and fine grained, and made up of small particles of 

 sand, which under the microscope are seen to be chiefly 

 quartz and microcline, with a smaller quantity of other 

 felspars, epidote, garnet, calcite and other minerals, em- 

 bedded in a very fine-grained argillaceous material. 

 This " sandy mud " contains a vast number of boulders 

 and pebbles of an immense variety of rocks, amongst 

 which are conglomerates, quartzites, sandstones, shales, 

 slates, crystalline limestones, jaspers and banded iron- 

 stones, granites and gneisses, diabases and amygdaloidal 

 lavas, quartz porphyries, serpentines, etc., the nature of 

 the inclusions varying in different localities. 



These boulders are, as a rule, scattered irregularly 

 through the matrix ; only in rare instances are they ar- 

 ranged in one or more well-defined layers. Plate IX., 

 from a photograph of the rock exposed in a ravine near 

 Prieska, gives a good idea of the manner in which the 

 pebbles and boulders usually occur. 



Not only is the great variety in composition and size 

 of the boulders remarkable, but a large proportion of 

 them have scratched and striated surfaces, while in 

 many there are one or more striated flattened faces 

 giving a roughly facetted form to the pebbles. In all 

 respects these boulders are similar in form and in the 

 nature of their striations to the scratched boulders that 

 are found in the moraines of modern glaciers and in the 



