466 JSTotices of Memoirs — A. R. Hunt — Start Point. 



Mr. A. Harker, F.G.S., on examining the sandstone, at once 

 pointed out the presence of tourmaline and white mica, of detrital 

 origin ; and considered that the rock had the appearance of having 

 been derived from a tourmaline-bearing granite. 



On a careful examination of two slides of the quartz -schist,^ the 

 author detected a single grain of tourmaline. Six additional slides 

 were forthwith prepared, and detrital tourmaline was found in them 

 all. One of these slides contains a pellucid grain of quartz with 

 fluid inclusions and active bubbles ; another contains a grain crowded 

 with hair-like inclusions and with one fluid inclusion whose bubble 

 is easily moved by the heat of a wax match. Both these grains 

 could be easily matched in the quartzes of different granites. 



The derivation of the quartz-schist from granites of more than 

 one character, but one of which must have been schorlaceous, seems 

 clearly indicated. 



The above facts have two distinct bearings, viz. as to the age of 

 the metamorphic schists of South Devon, and as to the derivatiou 

 of the tourmaline. 



The two rocks under consideration, viz. the quartz-schist and the 

 Devonian sandstone, are related to each other in four particulars, 

 insomuch as they contain four constituents common to both, viz. 

 detrital tourmaline, detrital mica, quartz of fine grain, and iron. 



It seems difficult to avoid the conclusion that such similar rocks 

 must be of like age and derivation ; and that as the sandstone is 

 undoubtedly Devonian, the quartz-schist, one of the metamorphic 

 schists of South Devon, must be of Devonian age also, and not 

 Archaean, as has been supposed by some geologists. 



The derivation of the tourmaline is a more difficult question. 

 Whatever may be the age of the mass of the Dartmoor granites, 

 those of a schorlaceous character seem to be post-Carboniferous. 

 Moreover, no tourmaline has been noticed in the granites trawled 

 in the English Channel. There is thus no recognized source of 

 pre-Devonian tourmaline in the neighbourhood of South Devon, yet 

 the source of derivation of the rocks under discussion could not 

 seemingly be reiBOte, or the tourmaline, quartz, and mica could 

 scarcely have kept together. The tourmaline granites of Cornwall 

 would meet the case, if any of these are of pre-Devonian age ; but 

 on this point the author has no information. 



Besides the tourmaline observed in the rocks at Street Gate and 

 Start Point, the author has noticed the same mineral, occurring in 

 the same way, in a sandstone from near Tinsey Head in Start Bay, 

 and in a sandstone from near Charleton on the Kingsbridge estuary, 

 both of Devonian age. 



1 The hand-specimen selected for slicing was kindly placed at the author's diposal 

 by Mr. A. Somervail, of Torquay. 



