100 THE ONGELUK VOLCANICS 



layers of red and grey jasper and chert, and greenish 

 flagstones. 



The lowest lavas rest upon thin-bedded hard shales 

 or upon the tillite of the Lower Griqua Town beds. The 

 variation in the thickness of the shales, and their ab- 

 sence in places, perhaps indicate an unconformity, but 

 so far as the general structure of the country is con- 

 cerned the unconformity is of little importance. The 

 Ongeluk beds have not been found lying directly on 

 rocks older than the tillite. The lavas are as a rule 

 easily distinguished from the others in the Colony by 

 their appearance. They are fine-grained rocks with a 

 devitrified matrix containing minute feathery microlites 

 of augite, or hornblende, small felspars, and small cry- 

 stals of enstatite or pseudomorphs after that mineral ; 

 the structure is of the kind called hyalopilitic. Amyg- 

 daloidal lavas, in which chalcedony, calcite and chlorite 

 fill the steam-holes, are not infrequent. The Ongeluk 

 lavas are less frequently amygdaloidal than those of the 

 Pniel series. Volcanic breccias and tuffs are occasion- 

 ally found. 



Along the Mashowing and Kuruman Rivers some of 

 the lava-flows have a peculiar kind of " pillow " structure, 

 due to the presence of large blocks of lava which are 

 separated from each other by a darker-coloured substance 

 with a laminate parting parallel to the surface of the 

 nearest block. The lava blocks may be as much as eight 

 feet wide ; they have rounded edges and corners, and 

 the surfaces of adjoining blocks have complementary 

 shapes. 1 The rock in the lava blocks is not amygda- 



1 Figures are given in G. C., xii., 65, 66. 



