Yol. 65.] PETROGRAPHY OP THE NEW RED SANDSTONE. 229 



15. A Contribution to the Petrography of the, New Red Sandstone 

 in the West of England. By Herbert Henry Thomas, M.A., 

 B.Sc, F.G.S. (Read March 10th, 1909.) 



[Plate XII.] 



Contents. 



Page 



I. Introduction 229 



II. The Minerals of the New Red Sandstone 231 



III. Physical Characteristics of the Finer Material 235 



IV. Mineralogical Characteristics of the Finer Material 



and Distribution of Mineral Species 237 



V. Conclusions 241 



I. Introduction. 



In a former paper 1 communicated to this Society I dealt with the 

 mineralogical constitution of the great Pebble-Bed, as developed in 

 Devon and Somerset. The results obtained encouraged me to 

 undertake a similar study of the other members of the New Red 

 Sandstone Series in that district ; and, in the following communica- 

 tion, I have endeavoured to point out the mineralogical and physical 

 differences exhibited by the microscopic particles which form the 

 finer portions of the deposits. The classification and subdivision 

 of the New Red Sandstone Series in Devon and Somerset has 

 gradually evolved from the labours of several geologists, and I 

 propose to use those subdivisions which are now adopted by the 

 Geological Survey. 2 



The New Red Sandstone in the area under consideration may be 

 divided as follows, commencing with the older beds : — 



(1) Lower Breccias and Lower (4) Upper Sandstones and 



Sandstones. 



(2) Lower Marls. 



(3) Pebble-Bed and Conglome- 



rate. 



Marls, with the Upper 

 Keuper Sandstones of 

 the Bridgwater dis- 

 trict. 



These subdivisions are founded solely on lithological characters, 

 and therefore form the most natural basis for a petrographical 

 study. 



The Lower Breccias are well-developed, and have been examined 

 on the south coast at Teignmouth and Dawlish, along the western 

 side of the Exe estuary on the flanks of the Haldon Hills, in the 

 Exeter district, and in the Crediton and Stogumber valleys. They 

 usually consist of deep-red breccias, sands, and sandstones, most 



1 ' Mineralogical Constitution of the Finer Material of the Bunter Pebble- 

 Bed in the West of England' Quart. Journ. Geol. Soc. vol. lviii (1902) p. 620. 



- W. A. E. Ussher, ' Geology of the Country around Exeter ' Mem. Geol. 

 Surv. 1902; see also ' Subdivisions of the Triassic Rocks, between the Coast, 

 of West Somerset & the South Coast of Devon' Geol. Mag. dec. 2, vol. ii 

 (1875) p. 163. 



Q. J. G. S. No. 259. r 



