part 4] the millstone geit of YORKSHIRE. 273 



Rutile. — All the colours previously described as found in the 

 grit are present in one or other of these separations. No geni- 

 culate or cordate twins have been found. 



Zircon. — As would be expected, these in nearly all cases show- 

 good crystal outline, of the forms described for the zircons in the 

 grit. Zonary structure is common, as are also inclusions of 

 various forms. 



Xenotime. — This is similar to the zircons in form, but is of 

 reddish or reddish-brown colour, and somewhat larger than the 

 majority of the zircons. 



Ilmenite and leucoxene. — As was noted in the grit, 

 ilmenite is by far the commonest of the iron-bearing minerals, 

 and is associated with leucoxene — with anatase- and rutile-crystals 

 as secondary outgrowths. 



Tourmaline. — The long pyramid is the common form, ter- 

 minated by rhombohedra in a few cases. The colour is brown 

 to purplish, and is strongly pleochroic. Blue tourmaline has been 

 noted in the separation from the schist. 



Apatite. — As stated, this mineral has not been recorded from 

 the grit, and is only of rare occurrence here, the form being 

 acieular. 



Topaz. — This occurs in only one of the granites, and is of 

 irregular form, with good basal cleavage developed. 



Monazite. — Its presence in these granites is most interesting. 

 The mineral is often rounded, but one good crystal has been 

 noted. 



An interesting and important light upon the question whether 

 the rocks of the Lake District contributed material to the 

 Millstone Grit has been given by the work of Mr. R. H. Rastall 

 & Mr. W. H. Wilcockson upon the accessor}" minerals of the 

 Lake - District granites. 1 They find that some of the most 

 abundant of the heavy minerals of the grit, such as rutile and 

 tourmaline, are by no means common constituents of these 

 granites, while sphene proves to be common in the Lake-District 

 granites and absent from the Millstone Grit. Apatite also is 

 common in these granites and absent from the grits, but it must 

 be noted that it occurs in three of the pebbles from the grit. 

 The crucial test, however, seems to be the monazite. "When the 

 paper above quoted was read before the Geological Society, Dr. H. 

 H. Thomas, replying to the discussion on behalf of the Authors, 

 stated that, although searched for spectroscopically, monazite had 

 not been detected. Apart, then, from the question whether the 

 granites of the Lake District could or could not, by reason of 

 their exposure, have contributed material to the Millstone Grit 

 as previously discussed, there are here definite reasons for 

 excluding them as possible sources for the material making up 

 the Millstone Grit Series of the Yorkshire area. 



1 Q. J. G. S. vol. lxxi (1915-17) pp. 592-622. 



