part -4] THE MILLSTONE GRIT OF YORKSHIRE. 265 



Under the microscope the larger grains are seen to be fairly 

 rounded, but the smaller ones are angular, this being due to the 

 extraordinary development of the dodecahedral cleavage (best 

 seen in the small garnets which occur in the shales of the Grit 

 Series, and it is also the characteristic feature of the garnets 

 that I have separated from the sandstones of the Coal Measures). 

 Dr. T. O. Bosworth l has noted a similar peculiar angularity in 

 the garnets found in the Coal-Measure sandstones in Scotland, 

 and the figures given by him could well be taken to represent the 

 garnet-grains of the Millstone Grit and Coal-Measure sandstones 

 of Yorkshire. 



Another remarkable feature is the skeletal or drusy development 

 shown by a very large proportion of the larger grains, the rhombic 

 faces of the small individuals that make up the grains being easily 

 observable. Deep pits with apparently rhombic section also occur 

 in the grains, though perforations are somewhat rare. 



Inclusions are very common in the larger grains, so much so that 

 the grains are seldom isotropic. The nature of these inclusions 

 has not been determined with certainty, but most of them are of 

 some mineral with strong double refraction, occurring either as 

 stumpy prisms, or long slender needles; while others, again, are 

 of irregular outline. 



Chemical and spectroscopic tests show that the garnets are 

 chiefly of the almandine type. 



Iron Pyrites. — This mineral is not nearly so abundant in the 

 Millstone Grit as in the Coal Measures. It occurs generally as 

 small cubes or rounded grains. 



M agnetite.— In none of the separations is this mineral 

 abundant It is mostly in the form of rounded grains, but some 

 of octahedral form have been found. As one would expect, it is 

 most abundant where ilmenite is also common. 



Anatase. — As separate grains which may be of primary origin, 

 tli is mineral is rare; but, as a secondary outgrowth from leucoxene, 

 it is abundant. The stages can be traced as follows: — ilmenite, 

 leucoxene, anatase, or rutile. It is colourless, and of tabular habit, 

 with low pyramids developed in some eases. 



Rutile. — In some separations this mineral is very abundant, 

 indeed, it is the most abundant heavy mineral in some of the 

 shales. It has proved to be invariably present. The colour 

 varies from light yellow, through deep amber and brown, to foxy 

 red. It may be said that the deep amber and brown are the most 

 abundant. Many crystals show an almost complete development 

 of the faeces with sharp edges. In other cases the prism-faces only 

 arc well developed, and the terminations irregular or rounded. 



1 • The Heavy Minerals in the Sandstones of the Scottish Carboniferous 

 Rocks' Proc. Geol. Assoc, vol. xxiv (1912) pp. 57-61 & pis. x-xi. 



