256 HE. A. GILLIGAX OX THE PETROGRAPHY OF [vol. lxxv, 



felspars, and showing micrographic structure, generally somewhat 

 fine. 



The quartz in most cases shows undulose extinction, and the 

 laminae of the micas and felspars are often curved. A notable 

 feature is the frequent presence of such accessory minerals as zircon 

 and rutile, included often in the quartz and felspar, but usually 

 in greatest quantity in the micas. This last point is of interest, in 

 view of the work which has been carried out by me upon the heavy 

 minerals of the Millstone Grit, and presented in a later part of this 

 paper. Perhaps the most noteworthy of these granite- pebbles is 

 that illustrated in PL XVII. fig. 4. which in the hand-specimen 

 shows a distinct gneissose structure; but, when examined in 

 polarized light, it is found to yield no evidence whatever of 

 crushing, and is a particularly fine example of a protoclastic granite 

 or gneiss, the intense foliation having been produced prior to its 

 final consolidation. 



The quartz- and felspar-porphyries are usually buff-coloured, 

 with the quartz and felspar (the latter often altered) well seen on 

 the broken surface. The porphyritic crystals are often much 

 corroded, especially the quartz, and the felspars are commonly 

 microcline or microperthite with a tendency to micrographic 

 structure as an outgrowth from some of the felspar-crystals. 

 Liquid inclusions with movable bubbles are abundant in the quartz 

 of a number of these rocks. The ground-mass is of the usual 

 microcrystalline type, in which small wdsps of biotite frequently 

 occur. One of these porphyries bore so striking a resemblance 

 to the well-known rhomb-porphyry of the Christiania district that 

 it was sent to Prof. W. C. Brogger for determination. He stated 

 that it was not a true rhomb-porphyry, but said that it was possibly 

 one of its facies, the rock being too much decomposed to make the 

 identification certain. 



The quartz-diorite pebble is the only one of basic type yet 

 found. 



(/) Pebbles of Metamorpliic [locks. 



Sorby records the finding of a single pebble of tine-grained 

 mica-schist or claj^-slate, and suggests that their scarcity is 

 accounted for by the ease with which such rocks would be broken 

 up during transport. I can but endorse Sorby's remarks upon the 

 rarity of pebbles of schist through the Grit Series as a whole ; I 

 have, however, been fortunate enough to obtain a number of large 

 pebbles of schists and gneisses from the Middle Grits of Silsden. 

 Indeed this has been the ' happy hunting-ground ' for the igneous 

 and metamorphic pebbles. 



The granitoid gneisses are composed of practically the same 

 minerals as the granites described above, only differing in the 

 structures produced by shearing. In fact, there is a regular grada- 

 tion quite noticeable when the sections are compared. The schistose 

 rocks form an important group. Generally, they may be described 



