32 Transactions of the South African Philosophical Society. 



with boiling water that poured over a land surface covered with tall 

 grasses ? Had the mud been hotter the silica would have been 

 fused, as when a hay-stack burns. This rock, then, would have been 

 similar in origin to the Italian peperino, and, as we shall see later, to 

 the Cave Sandstone. Whether Darwin's white rock is this, or is 

 merely a trachytic tuff of which several are described in the Challenger 

 Eeports, must remain uncertain. I think, however, taking into con- 

 sideration Darwin's mention of the fragments of granite ejected with 

 the other materials of the tuffs, that some of this peculiar white rock, 

 at any rate, is the same as the Cave Sandstone. 



Dunn, in 1874," drew attention to what he called peperino beds in 

 the strata overlying the Molteno beds. By this term he meant the 

 beds of ashes apart from the Cave Sandstone. Ash beds occur in 

 conjunction with the Cave Sandstone in some places, for instance, 

 in Matatiele, and the description of the peperino of Monte Albano is 

 strikingly like that of the Cave Sandstone in many respects. The 

 Italian peperino consists of a ground-mass of tuff, in which sometimes 

 angular fragments of rock are embedded. The ground-mass is light 

 grey, fine and earthy, somewhat rough to the touch, and often porous; 

 the drusy character shows that it was once saturated with moisture. 

 The cavities are filled with zeolites and calcite, the material of 

 which has been derived from the rock itself. The porosity, however, 

 is not a constant characteristic of the rock. Light, unaltered 

 spaces in the ground-mass alternate with darker patches, which 

 may be explained by the action of acid vapours, according to 

 Di Tucci.f 



Microscopical examination shows that the ground-mass of the 

 peperino is composed of minute particles of volcanic glass which 

 enclose numerous crystals of augite and leucite. These glass particles 

 are attached to each other by an indeterminate grey substance. 

 Besides the glass, there are larger particles of mica, augite, olivine, 

 magnetite, leucite, and fragments of basalt and leucitophyre, as well 

 as limestone. These latter rocks occur as the finest dust which can 

 only be recognised under the microscope, and also as larger blocks 

 which are sometimes of very great size. Plant remains also occur. 

 The peperino occurs in beds sometimes 800 feet thick, and is 

 divided sometimes by beds of loose ashes, though for the most part 

 it occurs as one great unstratified mass. 



Suppose, now, that the volcanic glass in the peperino was replaced 

 by dust composed of the triturated rock fragments torn from the 



* Report on the Stormberg Coalfields. 



f Saggio di studi geologici sui peperini del Lazio. Real Accad. dei Luicei, 

 Rome, 1879. 



