84 A. Heard — The Petrology of the Pennant Series. 



coal seam of Monmouthsliire, and the lower boundary by the No. 2 

 Rhondda coal seam.^ 



This petrological investigation is chiefly concerned with the 

 Pennants, east of the River TafE, but has been extended to certain 

 areas, such as G-arth Hill, west of the TafE. The rock-specimens 

 examined have been selected from various horizons at different 

 localities along the south crop of the coalfield, and from districts 

 as far north as Ferndale, in the Rhondda Valley. 



Immediately below the No. 2 Rhondda is a mass of sandstone 

 which so closely resembles the Pennants that it is impossible to 

 distinguish between the two in hand-specimens. These sandstones 

 from the upper part of the Lower Coal Series have been included in 

 the work for the purpose of establishing a relation between them and 

 the Pennants. 



Macroscopically, the Pennants may be described as compact 

 micaceous sandstones and flags. The colour in unweathered 

 specimens varies from grey to green, and is sometimes almost black 

 where there is a local concentration of carbonaceous material. 

 Frequently the rock is highly false-bedded, and small lenticular 

 coal-seams appear at many horizons, consisting of material which 

 is clearly detrital matter. Often the sandstones are massive, without 

 any concentration of coal, but in this massive rock silicified plant 

 remains have been found, including numerous large Calamites. 

 Bands of conglomerate and shales are exposed at certain horizons 

 and near the Taff thin sulphury coal-seams appear, 



Appeaeance in Thin Section. 



A large number of specimens of Pennant Sandstone, from many 

 different localities, have been sliced and examined. There is little 

 variation shown in the different thin sections. Under the micro- 

 scope the rock is seen to be a fairly even-grained sandstone, con- 

 sisting mainly of quartz grains set in a matrix, which is difficult to 

 determine microscopically. Generally an aggregate polarization 

 is shown. Secondly, white mica and quartz are the only minerals 

 which can be identified in the matrix. The secondary quartz is 

 occasionally found enveloping the original quartz grains. 



On the whole there is considerable uniformity in the size of the 

 quartz grains, the average diameter being about "3 mm. in the 

 normal sandstone. These grains are subangular, but tend to 

 become angular with a diminution in size. Much of this angularity 

 is probably due to a secondary deposition of silica. The quartz is 

 clear and remarkably free from inclusions. Undulose extinction 

 is rare in the smaller grains, although in the larger grains it is 

 occasionally visible. Chert is common throughout the series, its 

 brown or yellow colour and its cryptocrystalline structure affording 

 a ready means of recognition. Detrital colourless mica is very 



^ A. Strahan in "The Country around Newport": Mem. Geol, Surv., 

 1909, p, 28. 



