part 4] THE MILLSTONE GRIT OF YORKSHIRE. 253 



II. Petrography of the Millstone Grit. 



As was stated above, the grade of the material making up the 

 beds of this formation varies considerably from a conglomerate 

 with pebbles 2 inches or more in their longer axes to the exceed- 

 ingly line material making np the shales. From the coarser beds 

 very large numbers of pebbles have been collected and examined 

 microscopically, while sections of the finer grits and sandstones of 

 different areas have been prepared and, in a few eases, of the 

 shales also. The minerals of the finer-grained beds have also been 

 examined, after suitable preparation by crushing, washing, etc. 



(//) Work of Dr. H. C. Sorby. 



Dr. Sorby, in the paper already cited, says l : — 



' By far the commonest pebbles are of quartz like that forming- one of the 

 constituents of coarse-grained granites, but a considerable quantity are 

 evidently portions of various yellow, pink, grey, and almost black quartz- 

 rocks or quartz-schists. There are also many pebbles of white or brownish 

 orthoclase-felspar similar to that in coarse-grained granites, and it therefore 

 becomes interesting to ascertain for certain whether some of the quartz and 

 this felspar did not originally constitute a granitic rock. After much carelul 

 search, I have at length obtained extremely good proof of this fact, for I have 

 found a few pebbles of undoubted granite, some of which are composed of a 

 union of quartz and felspar like a coarse-grained granite, which are in every 

 respect similar to the quartz and felspar so abundant as pebbles of each 

 mineral alone. Besides such crystalline felspars there occur pebbles of fine- 

 grained felspathic rock with a few laminae of mica, and though usually much 

 decomposed it appears to have been some kind of eurite. I have also found 

 one pebble of a moderately fine and even-grained rock much like some varieties 

 of greenstone or a fine-grained syenite consisting of a white felspar and of a 

 dark-coloured mineral, both too much decomposed to be accurately identified. 

 It is also interesting on account of being the largest pebble I haA*e yet found. 

 for it is nearly 4 inches in circumference. With the exception of a single 

 pebble of very fine-grained mica-schist, which some would call clay-slate. I 

 have not found good examples of any other decided and well-marked rocks." 



(b) Pebbles of Quartz. 



Anyone making a cursory examination of the pebbles of the 

 conglomerate must agree with Sorby that quartz is the dominant 

 type, forming the 'hummer' stones of the quarrymen. They also 

 vary greatly in colour, among the varieties noted by me being- 

 black, rose, red, yellow, milky, blue or opalescent, and clear. 

 The varying colours are, of course, due to inclusions of different 

 types. The pebbles may be rounded or angular, but in all eases 

 they show glistening surfaces due to the deposition of secondary 

 silica. One striking fact with regard to their shape is the frequent 

 occurrence of double sphenoid forms. Such forms suggesi an 

 origin from a coarse gneiss or pegmatite. The microscope reveals 

 the fact that all these quartz-pebbles have been derived from rocks 

 that have been subjected to mechanical deformation, which in 



1 Proc. Tories. Geol. & Polytechu. Soc. vol. iii (1859) pp. 673 74. 



