258 A. R. Hunt — Granitoid Fragments in Culm. 



I had in my own collection of slides the whole chain of evidence 

 from beginning to end. 



My own series consists of one contact slide, granite-and-grit, from 

 near Bovey Tracey, one granite-and-grit from near Lustleigh, three 

 fragments from New Red of Labrador, two near Ideford, two near 

 Yeoford, and two, strangely enough, from the Brixham raised beach. 

 They are all members of one remarkable family. 



Comparing my own slides in the light of Mr. Salter's, I noticed 

 that both my contact slides had in their granite portions well-marked 

 crystals of idiomorphic brown tourmaline. On close examination 

 minute crystals of the same brown tourmaline could be seen 

 pervading the interstices between the quartz granules of the Culm- 

 grit portions of the slides. A similar invasion by tourmaline was 

 then noticed, both in Mr. Salter's pebbles and in my specimens from 

 the New Red conglomerates. In some cases the aluminous cement 

 of the quartz granules seemed transformed into tourmaline. The 

 tourmaline in the conglomerate fragments is chiefly a bluish-green 

 variety, whereas that in my contact slides is brown. 



Of Mr. Salter's specimens I cannot, of course, speak, nor have 

 I retained a single note of them. I cannot refrain from saying as 

 much as I have, as never before in my experience has the comparison 

 of notes thrown so much light into an absolutely dark corner. 



It is abundantly evident that all these tourmalinized grits are the 

 debris of the rocks once immediately covering the great granitic 

 areas of the West of England. Grits once plutonic, permeated at 

 high pressure by the mineralized waters which filled the fissures in 

 the granite with the well-known felspar-quartz-schorl veins. Such 

 action on a grit results in a very deceptive altered rock, which may 

 be anything between a sedimentary rock and a crystalline one, but 

 all with the common link of tourmaline running through them, and 

 tourmaline very unlike the types usually occurring in the granitoid 

 rocks, where crystallization has not been so much hampered by the 

 original minerals of the sedimentary rock ; while, in the veins, 

 crystallization has been quite unimpeded. 



In the case of a tourmaline vein, it is often easy to see that the 

 mineral is as original as the quartz accompanying it. A fissure 

 existed, and freshly crystallized minerals occupied the space. But 

 in the case of mineralized waters attacking the aluminous or 

 felspathic matrix of a grit, it is not easy to distinguish to what 

 extent the newly-formed tourmaline is derived from the pre-existing 

 felspathic element, or from tbe invading fluids, which, perhaps, in 

 a neighbouring vein prove themselves quite capable of forming 

 felspar quartz and tourmaline without any outside assistance ; 

 whereas the felspar alone could supply neither boron nor fluorine. 

 Tourmaline is truly a protean mineral occurring in various forms, 

 each form no doubt arising from its own peculiar originating cause. 



The non-schorlaceous granites of the Culm conglomerates, coupled 

 with the schorlaceous grits of the New Red conglomerates, suggest 

 quite a number of interesting problems. The Culm conglomerates 

 bear witness that no portion of the Dartmoor granite, or of its 



