part 4] THE MILLSTONE GRIT OF YORKSHIRE. 2(53 



(3) Mica. 



In the coarse grit-beds mica is not plentiful, but becomes an 

 increasingly important constituent as the fineness of grain of the 

 rock increases, until in the shales it becomes the dominant 

 mineral. The potash-micas are the most plentiful in the Hags and 

 sandstones, while the ferromagnesian micas are much more common 

 in the shales. It may be that much of the mica in the shales is 

 of secondary origin, as suggested by Mr. W. M. Hutchings for the 

 mica in the shales of the Coal Measures. 1 



The size of the mica-rlakes is important, ranging up to a quarter 

 of an inch in diameter in some of the very micaceous beds, such as 

 that on Keighley Moor ; while similar large micas were observed 

 in the shales obtained from deep boreholes at Leeds and Bradford. 



Inclusions in the Micas. 



These are very frequent in the ferromagnesian type, and, in 

 addition to the oxides of iron which occur along the cleavage- 

 planes, zircons are extremely common. These are surrounded by 

 the usual pleochroic haloes. It seems quite clear that the 

 excellence of the crystalline form of the zircons obtained from 

 the shales and grits can best be explained by supposing them to 

 have been carried along as inclusions in the micas, and only 

 released by the decomposition of the mica in the place where they 

 are now found. 



IY. Heavy Minerals of the Millstone Grit. 



From the commencement of my investigations I have been able 

 to identify many of the so-called ' heavy minerals ' in the ordinary 

 sections of the grit which I have had prepared, and this is notably 

 the case with the zircons, garnets, and tourmaline ; but, for the 

 more detailed examination of such minerals, recourse was had to 

 the following method of separation. The grit, sandstone, or shale- 

 specimen, weighing approximately 1000 grms., was pounded, but 

 not ground, in a mortar, and, when thus prepared, was placed upon 

 the coarsest of a series of sieves of 10, 30, (30, and 90 mesh to the 

 inch respectively, and very thoroughly shaken. It was found that 

 almost the whole of the heavy minerals were passed by the 1)0 

 mesh, this being especially true of the zircons, rutile, and monazite. 

 From some of the beds, garnets large enough to be retained above 

 the 60 mesh were numerous ; but, of course, this was known by 

 examination of the specimens with a hand-lens. After dealing 

 with some twenty specimens by separating the heavy minerals 

 from all the grades which passed the 10-mesh sieve, I found that 

 it was sufficient to examine the material in the following way : — 



Shales — material which passed the 90 mesh. 

 Sandstones — Do. do. do. do. 60 and 90 mesh. 

 Coarse grit — Do. do. do. do. 30, 60, and 90 mesh. 



1 Geol. Mag. 1890, pp. 263-316, & ibid. 1894, pp. 36-45. 



