part -A] PETROGRAPHY OF fORKSHIRE MILLSTONE GRIT. 251 



13. The Peteogeaphi of the Millstone Grit of Yorkshire. 

 By Alrert Gilligan, D.Sc, B.Sc, F.G.S., Lecturer in 

 "Economic Geology in the University of Leeds. (Read 

 May 21st, 1919.) 



[Plates XV-XVIII : Microscope-Sections.] 



Contents. 



Page 



I. Introduction 251 



II. Petrography of the Millstone Grit 253 



(a) Work of Dr. H. C. Sorby. 



(b) Pebbles of Quartz. 



(c) Do. of Felspar. 



(d) Do. of Pegmatite. 



(e) Do. of Igneous Rockn. 



(/) Do. of Metamorphic Rocks. 

 (g) Do. of Sedimentary Rocks. 



III. Examination of the Finer Material 259 



(1) Quartz. 



(2) Felspar. 



(3) Mica. 



IV. Heavy Minerals of the Millstone Grit 263 



Table of Heavy Minerals. 



V. Heavy Minerals in Pebbles found in the Millstone Grit . 272 

 VI. Lithological Characters of the Land-Mass from the 



Denudation of which the Material of the Millstone Grit 



was derived 275 



VII. Divisions of the Millstone Grit Series 275 



VIII. Mode of Accumulation of the Material 276 



IX. Position of the Ancient Land- Mass 277 



X. Physical Relief of the Ancient Land-Mass 281 



XI. Drift- or Current-Bedding in the Grit 283 



XII. Comparison with the Torridon Sandstone 284 



XIII. Climatic Conditions 287 



I. INTRODUCTION. 



The term 'Millstone Grit' is applied to a series of beds which 

 attain a thickness of 5500 feet in Lancashire, consisting of con- 

 glomerates, grits, sandstones, and shales with occasional thin 

 impure limestones and beds of chert. It occupies a position in 

 the geological sequence between the Voredale or Pendleside Group 

 and the Coal .Measures. For the purposes of this investigation 

 the base <>f the Millstone (Irit in West and Xorth-West Yorkshire 

 has been taken at the base of the [ngleborough (irit, following the 

 precedent of the Geological Survey as pointed out in the Memoir 

 on the [ngleborough District, 1890, pp. 71 75. Since this is ;t 

 coarse grit, easily traceable on account of its lithological characters 

 and the hold escarpments to which it gives rise, it forms a good 



