part 4] PETROGRAPHY OF YOKKSHIRE MILLSTONE (IRIT. 271 



The heavy minerals may be classified, according to the type of 

 rock which is likely to have yielded them, as follows : — 



Garnets — crystalline schists, gneisses, granulites, and granites. 



Magnetite — crystalline schists, gneiss, and many igneous rocks. 



Rntile — granite, crystalline schists. 



Zircon — crystalline schists, gneisses, syenites, acid eruptive rocks. 



Xenotime — granites and gneisse>. 



Tourmaline — granites and pegmatites, and, as shown later, schists. 



Topaz — granites . 



Monazite — granites . 



Cordierite — crystalline schists (since, however, this is so rare a mineral 



in the grits, the evidence is of little importance). 

 Ilmenite and its derivative leucoxene— scattered through igneous rocks, 



usually of basic type. 

 Anatase — This is, I believe, mostly of secondary origin, as stated above. 

 Brookite — granites and crystalline schists. 

 Iron pyrites — this is chiefly of secondary origin in the grits. 



The most interesting of these minerals is undoubtedly the 

 monazite, which, as shown in the lists, is present in almost all 

 the beds examined, but chiefly in the coarser type, and most of 

 all in the Rough Rock. When it was first determined to be 

 monazite (February 1917) l the only other records for the British 

 Isles, so far as I am aware, were those made by Sir Henry A. Miers 

 for Cornwall and by Sir Jethro Teall for Loch Dee Granite. 2 

 It seemed to me, therefore, that here might be a crucial test for 

 the theory of its derivation from a northerly source, and I find 

 that Dr. W. Mackie has obtained this mineral in a large number 

 of separations from granites of Northern Scotland. I have also, 

 through his kindness, had the opportunity of examining a remark- 

 able series of his slides showing the monazite and other minerals 

 which he has so obtained. Dr. W. Mackie has given me his 

 permission to quote his remarks, and they are here appended : — 



' I have not so far published anything- regarding monazite in the rocks of 

 the North of Scotland, but I have quite a number of observations regarding 

 that mineral which have been accumulating for the last couple of years 

 or more. I find it unexpectedly widely distrib\\ted in the granites 

 of the North of Scotland ; thus, out of 32 specimens of granite examined in 

 this connexion, I find that monazite is present in 25. xenotime in 23. both 

 minerals being found together in 18. It is relatively abundant in the younger 

 granites, e. g. in the red granites of Elginshire, while I have found it in some 

 of the Shetland granites, e. g. that of Nesting. It is also present in the 

 Central Grampian granite, though not so abundantly in the specimens that I 

 have hitherto examined ; but perhaps the richest example that I have so far 

 seen was in a sample from a xenolithic (probably older) granite included in 

 the Central Grampian granite, which I took many years ago from the summit 

 of Cairngorm. I find it in the grey granite of Aberdeen (Rubislaw i. which is 

 presumably one of the older granites. It occurs in many granites in well- 

 defined crystals, in others in rounded or ovoid grains with rather corroded- 

 looking surfaces. ... In clastic rocks, e. g. in our local Upper Old Red 

 Sandstones, I have found monazite, but so far not abundantly.' 



1 I am indebted to Prof. H. L. Bowman for his kindness in making con- 

 firmatory spectroscopic tests of this mineral. 



2 'Silurian Rocks of Britain, vol. i— Scotland ' Mem. Geol. Snrv. 1899, 

 p. 619. 



