part 4] THE MILLSTONE OltIT OF YoKKSHIKE. '2i>7 



not lie in the same plane, and hence it requires a careful search by 

 racking the microscope up and down for each crystal to examine 

 them adequately. 



The zircons are for the most part water-clear, though some are 



cloudy. Associated with the zircons, as regards form and optical 

 properties, arc some reddish and reddish-brown crystals which arc 

 possibly xenotime. These are often clouded with alteration- 

 products. 



Calcite. — In some of the separations made from the Rough 

 Rock derived from a borehole at Mean wood, Leeds, considerable 

 quantities of calcite were obtained by using liquid of sp. gr. 2'7. 



In the microscopic sections of the same beds the calcite was seen 

 to form the matrix in a large part of the section. It is worthy of 

 note that the calcite was found to be most abundant, as one 

 would naturally expect, in proximity to shales yielding a marine 

 fauna. It cannot be said that calcite commonly acts as a matrix 

 to the grit ; and it has not been found in any quantity, except 

 under the conditions mentioned above. 



Ilmenite. — -Of the iron-bearing minerals this is by far the 

 commonest, if we exclude the limonite which is so abundant as 

 a matrix and the grains of clay-ironstone. In nearly all cases it 

 forms the core of an aggregate of leucoxene, anatase, and rutile 

 crystals, to which it has given rise. 



Tourmaline. — This mineral is of very common occurrence 

 throughout the ^Millstone Grit Series. Good crystal outline has 

 frequently been observed, and is in all cases that of the prism 

 terminated by rhornbohedra. The grains have often been so worn 

 as to be almost spherical. The colour is very variable, but some 

 shade of brown is the dominant one. Blue tourmaline is fre- 

 quently present; the colour, however, is seldom very intense. The 

 pleochroism of the brown variety is strong, while that of the blue 

 is much weaker. 



Inclusions are not common, but (when present) are small and 

 indeterminable. 



Barytes. — Like the calcite this acts as the matrix in many 

 cases, and has only been separated in any quantity from the grit 

 of one locality, namely, Great Almscliffe Crags (Kinderscout Grit > 



of Wharfedale. 



Brookite. — Only one undoubted grain of this mineral has been 

 found. It has a pale-yellow colour, and the pleochroism is weak. 

 The striatums parallel to the vertical axis are well developed. 



M ona/.ite. — Of all the heavy minerals this is certainly the most 

 important and interesting, its presence had long be. 'ii Buspected 

 in the separations from tin Rough l'ock ; but, when the gameti- 

 ferous layers were found and separations made, the honey -\ ellow 

 grains of monazite were so plentiful that its identity could be 



