part 4] PETKOGKAPHY OF YOEKSHIEE MILLSTONE GRIT. 279 



In the Geological Surve} r Memoir on the Yorkshire Coalfield, 

 1878, the diagram (pi. i, facing p. 32) showing the variations in 

 the grits when traced from Ashover (south-west of Chesterfield) to 

 the neighbourhood of Skipton, makes the fact of the increase of 

 thickness to the north very clear. 



The only possible conclusion to which we can come from the 

 facts set forth above is that the material of the grit series was 

 derived from some northerly source. 



The central barrier of St. George's Land, as it has been termed, 

 must be excluded as a possible source for more than a small 

 fraction of the material in the grit, for the following reasons : — 



(a) It has not the necessary lithe-logical character. 



(b) The area is altogether inadequate. 



In considering (b) it must be remembered that a large portion 

 of the barrier which existed in Lower Carboniferous times had 

 been covered by the encroachment of the Carboniferous Limestone. 

 This central barrier precludes the possibility of derivation from 

 the south of it. 



The Lake District has also been cited as a possible source of 

 supply of material, and it has been suggested that the large 

 felspars of the grit have been derived from the Shap Granite, 1 

 which is quite erroneous. The recent work of Prof. E. J. Garwood 2 

 in mapping and zoning the Carboniferous Limestone of the North 

 of England shows that the Lake District represented an area of 

 subsidence in Lower Carboniferous Limestone (Avonian) times 

 and was submerged beneath the waters of the so-called Avonian 

 ocean. 



In Scotland, land was to be found in the Southern Uplands in 

 Lower Carboniferous times, and, according to J. G. Goodchild, 

 material derived from this area is to be found in the beds of the 

 Basement Carboniferous of the North of England. When, how- 

 ever, we enquire how much of the area was exposed in Millstone 

 Grit times after the covering of Lower Carboniferous rocks had 

 been laid down, it is found to be quite inadequate to supply any 

 large proportion of the Grit Series of Yorkshire, though it possibly 

 contributed to the homotaxial deposits farther north. Here, again 

 also, in applying the lithological test, one is bound to conclude 

 that it rules out this area : for, while the large quartz-pebbles of 

 the coarse grits and some of the micaceous material of the shales 

 could have been derived in part from (or, perhaps it would be better 

 to say, resemble some of) the rock-constituents of the Southern 

 Uplands, the pebbles of fresh felspar and pegmatite found so 

 abundantly in the coarse beds of the Millstone Grit most certainly 

 did not come from that district. Also the granites forming the 

 highest points in Galloway are notably rich in sphene, a mineral 

 not found so far in the Grit. 



By this process of elimination, I am forced to the conclusion 



1 R. D. Roberts, 'Introduction to Modern Geology' 1893. p. 244, 



2 Q. J.G.S. vol. lxviii (1912) p. 553. 



