A. Heard — The Petrology of tlie Pennant Series. 91 



in it a large proportion of chlorite flakes, exactly like the detrital 

 chlorite of the Pennant Sandstone. The very few flakes of biotite 

 found show little trace of alteration. The paucity of heavy minerals 

 indicates that the Pennant sediments had not a granitic source ; 

 they were mainly derived from pre-existing sediments which were 

 rich in chert, had abundant chloritic material present, and yet were 

 capable of furnishing fairly fresh biotite. Metamorphosed shales 

 would account for the presence of cordierite and corundum. 



The false-bedding of the sandstones, the occasional rapid 

 alternation from conglomerate to sandy shale, and the entombment 

 of large amounts of vegetable material in the strata suggest that the 

 Pennant Sandstones were deposited under estuarine or lagoon 

 conditions. The adjoining shores were probably covered with a rich 

 vegetation which contributed material such as spore-cases, twigs, and 

 branches to the waters of the lagoon. 



The lenticular patches of detrital coal, and the conglomerates 

 with ironstone and coal j^ebbles, jjoints to the fact that areas of 

 Lower Coal Measure age were undergoing denudation. 



The Welsh Coal Series thickens considerably in a south-west 

 direction, and reaches a maximum in south-west Glamorganshire. 

 This increase in thickness is due, not only to a thickening of 

 individual sandstone and shale bands, but also to a south-west 

 incoming of fresh strata and more coal-seams.^ This suggests that 

 the source of the Pennant Series must be sought in that direction, 

 i.e. south-west (approximately). 



The lenticles of coal and shale, etc., which are formed through 

 current bedding, have, in the majority of cases, a long axis in a north- 

 east — south-west direction. Moreover, the steep side of each portion 

 of false-bedding generally faces the north-east ; apparently the 

 currents which carried these sediments came from the south-west. 



The Probable Source. 



The great development of schalstein and spilites in Cornwall, 

 beginning in the Middle Devonian and attaining a maximum in 

 Lower Carboniferous times, was only a westward prolongation of a 

 great mass of pillow lavas, extending from Moravia across the 

 Rhine into England.^ There is no reason to suppose that these 

 spilites did not occur further westward than the Cornish shores, 

 and as the spilitic suite of rocks are characteristic of geosynclinal 

 action, it is jjrobable that a land-mass extended parallel to this 

 line of spilites, which were formed in a trough running approximately 

 east and west. 



Spilites are characteristic of continued subsidence, and do not 

 accompany important uplift, yet those of Devon and Cornwall 

 do not extend into a horizon higher than the Lower Carboniferous. ^ 



^ Strahan, in " The Country around Swansea" : Mem. Gzol. Sttrv., 1907, 

 p. 33. 



2 Jukes-Browne, Stratigra phical Geology, 1912, p. 309. 



