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broken down and partially reconstructed porphyritic felsites, and considers that 
some lenticular aggregates which they contain may be looked upon as broken- 
down porphyritic felspars. Twenty-five years ago Dr. Callaway recognised the 
felsitic origin of these rocks, and the present investigations fully confirm his 
conclusion.! Similar structures have now been found in the rocks of the central 
area, on its western coast near Trecastell. These schists, therefore, may be 
looked upon in general as derived from acid igneous rocks. In other parts of 
both areas, however, mica-schists in continuity with them are found in intimate 
relations to schists of sedimentary origin, so that it is probable that pyroclastic 
material was present in the original igneous series. 
7. The Millstone Grit of Yorkshire: Some New Evidence as to its 
Source of Origin. By AtBerr Ginuican, B.Sc. 
More than fifty years ago Dr. H. C. Sorby attempted to trace the source 
whence the material which 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, uninteresting series 
of rocks. The late Mr. A. Longbottom, B.A., of the N igerian 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 High- 
lands, Numerous pebbles of felspar have been examined by the author and in 
each case found to be perfectly fresh microcline, the cross-hatching being 
beautifully clear. Pieces of pegmatite, the constituents being quartz and 
microcline, are very common in all the beds, but most abundant in the Kinder- 
scout Grit and Rough Rock. Some fragments obtained from the Plompton 
Grit at Knaresborcugh proved to be a peculiar silicified odlitic rock, the out- 
lines of the odlitic grains being traced out by small rounded bodies stained red 
or brown. A few pebbles show undoubted traces of organisms such as sponge 
spicules, &c. 
The heavy minerals of the grit are not numerous, the most plentiful being 
zircon and garnet. The felspars in the grit, both large and small, are quite 
fresh when first exposed, and this suggests either disintegration of the parent 
rock by differences of temperature and rapid transportation, or comparative 
absence of carbon dioxide in the atmosphere. The author has been much 
* See Brit. Assoc. Report, Manchester, 1887; also Q.J.G.S., 1897 and 1902. 
