A. Heard — The Petrology of the Peimant Series. 89 



These fragments became more interesting when viewed between 

 crossed nicols. The spaces between the walls were filled with 

 a colourless mineral, which compensated with the quartz wedge, 

 was insoluble in nitric acid, and gave the quartz polarization 

 colours. It was concluded that this was quartz. This mineral did 

 not exist as a mosaic, but a complete crystal of quartz filled each 

 cavity. 



The Conglomerates. 



The bands of conglomerates appear mainly in the vicinity of the 

 No. 2 Rhondda, although they occur locally at almost every horizon. 

 In the Rhondda Valley near Ferndale there is a conglomerate con- 

 sisting of ironstone and coal pebbles, quartzite pebbles, and plant 

 remains replaced mainly by iron carbonate. Sometimes the con- 

 glomerates were overlaid by sandstones or lenticular coal-seams, 

 and in places were immediately covered by sandy shales. Numerous 

 pebbles from the conglomerates were sliced, but all were very similar, 

 the most striking feature being the great proportion of large quartz 

 pebbles, all of which were very strained, showing strong undulose 

 extinction. These pebbles appear to have been derived from rocks 

 which had been subjected to great earth movement, and at one 

 time probably formed part of some pre-Cambrian ridge. Pebbles 

 of shale, chert, sandstone, and quartzite have been found in these 

 conglomerates. 



The occurrence of coal pebbles is significant, and suggests that 

 coal-seams were already undergoing denudation at this period. The 

 cement of these conglomerates varies from almost purely 

 recrystallized quartz to a sandy and carbonaceous matrix. 



The Shales. 

 The shales, which occur at many horizons, may be described as 

 sandy shales. They contain a large proportion of subangular quartz 

 grains, and abundant white mica. They appear to contain a larger 

 proportion of calcium and iron carbonates than the sandstones, 

 and also more pyrrhotite, pyrites, sphasrosiderite, corundum C?), 

 zircon, and rutile. In addition to the forms described above, 

 rutile needles have been found in the shales. Green pleochroic 

 cleavage flakes of chlorite are abundant in every specimen examined. 

 These shales are petrologically remarkably similar to the sandstones 

 of the Pennants, except that the shales appear to contain a higher 

 heavier mineral content. The heavy minerals are found concentrated 

 in layers, different parts of the same specimen containing practically 

 none, or abundant, heavy minerals. 



The Clays. 

 Lenticular patches of clay have been found amongst the sand- 

 stones, partioularly in a quarry due south of Caerphilly in the south 

 crop. These reddish clays contain iron in the form of oxide, which 

 can be observed under the microscope as a pellicle on the minute 



