part 4] THE MILLSTONE UIUT OF YOJIK.SHIHE. 26 L 



Further, Dr. Mackie remarks : 



' It may be stated as a fairly general law that acicular and irregular 

 inclusions pre-eminently abound in the quartz of granite ; that the regular 

 group is to be found in various proportions, but always in relatively large 

 numbers, in the quartz of gneiss and the younger schistose rocks.' 



He also states that, in his work on the Scottish rocks, inclusions 

 of different kinds occur frequently in one and the same grain, and 

 then such a grain i s classed under the heading of 'acicular' or 

 ' regular ' if these forms are present at all. 



Among the minerals of good crystal form included in the quartz 

 of the Millstone Grit, and termed ' regular ' inclusions, are the 

 following (in decreasing order of relative abundance) : — zircon, 

 tourmaline, rutile, and oxides of iron. 



The acicular inclusions appear to be almost invariably rutile, 

 though tourmaline and sheaf-like aggregates, which may be 

 sillimanite, have been rarely seen. 



In their order of relative abundance, the inclusions may be 

 classed as follows : — -regular, irregular, acicular, negative. (These 

 last are almost negligible). 



It follows from Dr. Mackie's determinations that the deduction 

 from the inclusions is in agreement with that previously made : 

 namely, that the quartz has been derived for the greater part from 

 rocks which have been subjected to pressure. Again, the frequent 

 occurrence of streams and rows of the irregular inclusions is a 

 feature which reminds one of the inclusions in such a rock as the 

 Lewisian Gneiss, of which I have had sections prepared for 

 comparison. 



Sir Jethro Teall states, regarding the inclusions in the blue 

 quartz of the pyroxene-gneisses with quartz of the North- West 

 Highlands, 1 



'These are of four types : (1) rows of minute dots ; (2) extremely thin hairs, 

 formed in all probability of the same substance as the dots ; (3) cavities with 



very dark borders ; and (4) minute irregular flecks and grains A similar 



blue quartz occurs in the grits of the Southern Highlands, and there also it 



contains the minute hairs and rows of dots it seems probable 



that this colour is due to the inclusions.' 



All the types mentioned by Sir Jethro Teall are present in the 

 quartz of the Millstone Grit; and further, as was stated previously 

 in the description of the larger quartz-pebbles, liquid inclusions, 

 usually hexagonal or coffin-shaped and with movable bubbles, are 

 common. The relative size of the bubble to the fluid-cavity differs 

 greatly in the various grains, which points to the fact, as Sorby has 

 shown, that they have been derived from rocks which have solidified 

 at different temperatures and pressures, the ratio of bubble to 

 cavity being of course greatest in those which have solidified at 

 the greatest depths, and therefore probably from the highest tem- 

 perature. The highest ratio of bubble to cavity is about' 0'25, 



1 'North-West Highlands of Scotland ' Mem. Geol. Sun. 1907, p, 54. 



Q. J. G. S. No. 300. x 



