362 

 THE MILLSTONE GRIT OF YORKSHIRE 



Some New Evidence as to its Source of Origin. * 



ALBERT GILLIGAN, B.Sc, F.G.S. 



More than fifty years ago Dr. H. C. Sorby attempted to trace 

 the source whence the material wliicli makes up the Millstone 

 Grit had been derived, by making a collection of pebbles which 

 occur so abundantly in some of the beds. Among these he 

 found some small fragments of mica-schist, quartz-schist, 

 and a few pebbles of undoubted granite. The largest pebble he 

 obtained was about four inches in circumference and of a type 

 resembling a fine-grained syenite or greenstone, but too much 

 decomposed to be accurately identified. The pieces of granite 

 were composed of quartz and felspar, suggesting by their 

 appearance derivation from coarse-grained granites. Pebbles 

 of quartz he found to be commonest, and he also described 

 some pieces of white or brownish orthoclase felspar. 



The granites he found were quite unlike any with which he 

 was acquainted in the British Isles, being too coarse and much 

 more like those of Scandinavia. Further, the current bedding, 

 which Dr. Sorby examined over an area of twenty-five square 

 miles, pointed to a drifting from the north-east, and he therefore 

 suggested some south-westward prolongation of an ancient 

 Scandinavia as the source of origin of the material making up 

 the great mass of the Millstone Grit of Yorkshire. Since this 

 early work by Dr. Sorby nothing has been added to our 

 knowledge of the lithology of this, to most people, uninter- 

 esting series of rocks. The late A. Longbottom, B.A., of the 

 Nigerian Survey, collected some very large pebbles from the 

 Middle Grits of Silsden. These have been examined by the 

 author, who has also extended his researches into the other 

 beds of the series in various parts of Yorkshire. Some of the 

 pebbles are of a very large size, one obtained from Netherwood 

 Plantation Quarry, Silsden, measures 10 inches by 8 inches by 

 3 inches, and is a reddish granitoid rock with large porphyritic 

 felspars. The pebbles show a remarkable assemblage of rocks,, 

 igneous, sedimentary, and metamorphic all being represented, 

 but by far the commonest are acid igneous rocks — granites, 

 quartz and felspar porphyries. Only one specimen of basic 

 igneous rock has been found. The metamorphic rocks are 

 quartz-schist and mica-schist with a few fragments of gneiss. 

 One of the mica-schist pebbles has been identified by Mr. 

 Barrow as similar to a rock described by him occurring in the 

 Moine Schists of the East Central Highlands. Numerous 

 pebbles of felspar have been examined by the author and in 



* Read at the Meeting of the British Association at Dundee. 



Naturalist. 



