TRANSACTIONS OF SECTION C. 643 



nature as that of tlie \'olcai:ic materials with the unaltered slates to the north. 

 From these facts it seems evident that the chloritic series is nothinjy more than a 

 Devonian volcanic group, of which the Torcross, Stoke Fleming, Dartmouth, and 

 Kingswear coast tuii's and diabases were either sporadic otishoots or evidences of 

 more or less contemporaneous local vulcanicity. 



The more evident crinlsling of the mica schists in contact with the chloritic 

 group seems to be due to their comparative softness and greater fissility during the 

 crumpling and contraction to which both were subjected. 



The comparative suddenness of the transition from unaltered to more or less 

 highly altered rocks may be explained by the lessening of strain (in receding from 

 the harder masses of ancient rocks, against which the beds were jammed), being 

 coincident with the thinning out of the volcanic materials northward, and further- 

 more favoured by the soft character of the grey slates with limonitic interfilmings 

 which evexywhere bound the northern chloritic band on the north. It is not the 

 author's present purpose to enter more particularly into the stratigraphy of this 

 interesting region, which is not yet thoroughly worked out. It only remains to 

 acknowledge the prior claim of Mr. Somervail to the suggestion of the identity of 

 the Devonian diabases with the chloritic rocks.^ 



6. On the Occurrence of Detrital Tourmaline in a Quartz-scliist ivest of 

 Start Point, South Devon. By A. R. Hunt, M.A., F.G.S. 



While examining the Devonian clift's near Street Gate, at the north-east end of 

 Slapton Sands, South Devon, in company with Mr. W. A. E. Ussher, F.G.S. , the 

 author selected a hard micaceous sandstone of fine grain, occurring as a band 

 between softer rocks, for comparison with a micaceous quartzite or quartz-schist, 

 previously noticed by Mr. Ussher at a point on the coast south of Start Fai-m and 

 west of Start Lighthouse. The quartz-schist occurs as an impersisteut band among 

 the mica-schists west of 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 distmct bearings, viz., as to the age of the meta- 

 morphic schists of South Devon, and as to the derivation 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 



' The views above expressed are those to which the author himself has been led, 

 but they have not yet been fully considered and adopted by the Geological Survey. 



^ The hand specimen selected for slicing was kindly placed at the author'* 

 disposal by Mr. A. Somervail, of Torquay. 



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