part 4] THE MILLSTONE GRIT OF YORKSHIRE. 275 



It is, moreover, worthy of note that in the coarse beds the larger 

 grains (quartz, felspar, etc.) are frequently well rounded, while the 

 smaller grains are always angular. 



VI. LniioLooiCAL Characters of the Land-Mass from the 

 Denudation of which the Material of the Millstone 

 Gbit was derived. 



The first and by far the most important rock making up this 

 ancient land-mass must have been a granitoid gneiss of very acid 

 composition, which was ramified by veins of pegmatite. Unaltered 

 granites mu>t also have been present, but the evidence of crushing 

 found in the great mass of the quartz of the Millstone Grit makes 

 it quite certain that they only formed a subordinate portion of the 

 whole mass. The dominant felspar in these rocks must have been 

 microcline, the large pebbles of this felspar being derived from the 

 pegmatites. The presence of mica-schist and quartz-schist, and 

 also the abundance of mica in the shales, sometimes making up 

 beds 2 to •'{ feel thick of almost pure white mica, make it safe 

 to infer that schists of various types constituted a large portion 

 < f the land. The garnets which are so common would in large 

 part have been yielded by the schists. The volcanic rocks, quartz, 

 and felspar-porphyries are significant as showing the presence of 

 dyke- of acid character, which were to be found traversing all or 

 any of the other rocks present. 



The abundance of chert-pebbles (sometimes showing oolitic 

 structure; with cryptocrystalline and microcrystalline silica, often 

 witli rhombs representing carbonates, enable us to draw the in- 

 ference that limestones containing chert-bands, and also in part 

 silicified. were associated with the igneous and metamorphic rocks. 

 Other sedimentary rocks, such as grits, sandstones, and mudstones, 

 must also have been present, as pebbles of such rocks occur in 

 the grit. Some of the rounded quartz-grains found in the grit 

 may possibly be of contemporaneous origin; but 1 am strongly 

 disposed to think that the greater number have been derived from 

 pre-existing grits and sandstones, and therefore such types must 

 have formed part of this ancient land-mass. 



VII. DlAISIONS OF THE MILESTONE GlUT SERIES. 



The area occupied by the Millstone Grit Series from Derbyshire 

 to Yorkshire was surveyed under the direction of the late Prof. 

 A. H. Green (who formerly occupied the Chair of Geology at 

 Leeds University). In the Memoir of the Geological Survey on 

 the Yorkshire Coalfield, 1878, the following observations are re- 

 corded as a result of the work of the Survey (p. 27) : — 



' Subdivisions of the Millstone Grrif in the North Central Counties. 



Rough Book Almost invariably a coarse and massive felspatbic grit about 

 100 feet thick. A band of flagstone frequently at the base. Called 

 also First Grit. 



Shales. 



