part 4] THE MI U, STONE OlMT OF YORKSHIRE. 255 



which he found, and evidently believed all the large felspars and 

 that associated with the quartz in his ' coarse-grained granite ' to 

 have been orthoclase. I have stated above, when describing the 

 felspar-pebbles, that orthoclase is either extremely scarce or absent 

 as large pebbles. Similarly, an examination of these pegmatites in 

 thin sections under the microscope shows that the felspar is, with 

 one exception, microcline or microcline-microperthite. The one 

 exception is, however, of extreme interest. It is the largest piece 

 of pegmatite yet found and was presented to me by Mr. J. Holmes 

 (of Crossbills), to whom I am also greatly indebted for other 

 valuable assistance. Before being broken it measured 5 x 2| x 2| 

 inches, and Avas found in the Middle Grits near Silsden. The 

 felspar in this case is albite, and so the rock would be termed a 

 soda-aplite. The quartz associated with the felspar in these 

 pegmatite pebbles is usually blue or opalescent, and shows undulose 

 extinction, but not the mylonized structure so common in the large 

 quartz-pebbles. Mica (muscovite) is only sparingly present in 

 these pebbles. The pebble which Sorby named a eurite would 

 almost certainly belong to this group. 



(e) Pebbles of Igneous Rocks (other than those of the 

 Aplite-Pegmatite Family). 



A remarkable assemblage of pebbles consisting of igneous and 

 metamorphic rocks has been obtained from the Middle Grits and 

 Rough Rock of the Aire Valley. Many of these pebbles measured, 

 before being broken for examination, as much as 6XoX l.j inches, 

 while one measured 10x8x4 inches, and is the largest yet re- 

 corded from the Millstone Grit. Occurring as they do in beds 

 which are fine-grained, it is interesting to enquire as to the manner 

 in which they were transported. The bed in which they occur is 

 crowded with plant-remains, and I was told by the workmen that 

 numerous trunks of trees had been unearthed during quarrying 

 operations. This would seem to afford a sufficient explanation, the 

 pebbles having been carried down by drifting vegetation. The 

 remarkable angularity of some of the specimens would also lend 

 support to this theory; but it is not so easy to explain how the 

 plants of the Carboniferous Period such as Sigillaria, Lepido- 

 <lf ml ron, etc., with the long horizontal roots, could have obtained so 

 strong a bold upon these large blocks as to transport them for any 

 great distance, such as I conceive these must have travelled. The 

 problem is similar to that of the occurrence of the quartzite and 

 granitic pebbles and boulders found in coal-seams, and I believe 

 that the agency suggested above explains both cases. 



The types of igneous rocks represented arc the following : — 

 Granites, quartz- and felspar-porphyries (several specimens of each) 

 and quartz-diorite (one specimen). 



The granites are always pink or flesh-coloured and fine-grained, 

 no porphyritic crystals having been present in any of them. The 

 usual type is a two-mica granite with orthoclase and plagioclase 



