part 4] THE MILLSTONE GRIT OF YORKSHIRE. 289 



most active in causing the break-up of a granitic rock by an 

 examination of the felspars found in deposits resulting from such 

 action. If the felspars are fresh and unaltered, the inference is 

 that the break-up of the original rock has been brought about by 

 mechanical means. The freshness of the felspars in the deposits 

 at the foot of the Himalayas and in the Gangetie basin they 

 believed was due to the disintegration of the parent rock by frost 

 and ice. J. W. Judd. in his descriptions of the Xile deposits, 

 speaks of the freshness of the felspars, 1 and points to the area from 

 which they have been derived as undergoing disintegration by the 

 unequal expansion of the minerals of the rocks consequent upon 

 the heat of the sun by day and the subsequent cooling by radiation 

 at night. From the commencement of my investigations the 

 exceeding freshness of the felspars of the Millstone Grit has been 

 one of the main points which has been quite clear. Many of the 

 photographs accompanying the present paper show this, and there 

 can be no question of subsequent formation within the grit itself. 

 Even in the minute fragments found in some of the finer-grained 

 beds, which would, of course, be more easily affected than the 

 large ones on account of the proportionately larger surface, many 

 of these plagioclase felspars show no trace of decomposition. The 

 microcline is indeed well known to be the most resistant of the 

 felspars : in all the grits this felspar occurs, both in the large 

 pebbles and in the small grains, in the most perfectly fresh condition, 

 and when a large pebble is fractured the cleavage-faces are often 

 as bright and lustrous as possible. The kaolinization noticeable in 

 some of the grit-beds, as in the Rough Rock of Horsforth, has 

 evidently been brought about since the rock has been exposed in 

 post-Carboniferous times, the original outline of the felspars being 

 usually traceable. 



When discussing the source of the calcium carbonate in the Car- 

 boniferous Limestone, I pointed out that much of it must be due 

 to leaching of calcareous material from silicates of the alkaline 

 earths : this would necessarily mean an abstraction of the carbon 

 dioxide from the atmospheric-oceanic supply, causing a considerable 

 diminution in that substance. This would leave the Millstone 

 Grit period with a much depleted supply, so that the freshness of 

 the felspars in the Grit and the formation of the Carboniferous 

 Limestone are in consonance one with the other, chemical agents 

 being active in the formation of the latter and of little importance 

 in the former. In fact, this period may be looked upon as one 

 when the atmospheric-oceanic supply was being replenished. 



As Judd, in the paper above quoted, pointed out, an examination 

 of the clays (if modern deposits) or shales (if ancient deposits) 

 should yield evidence of the amount of chemical decomposition which 

 the parent rocks have undergone. If kaolin is the chief constituent, 

 then chemical action has been responsible to a great extent; but, if 



1 Proc. Roy. Soc. vol. xixix (1886) pp. 215-17. 



