J. H. Teall — Cheviot Quartz-Fehites and Augite- Granites. 107 



It is necessary to state that my visit to the Cheviots was nciacle 

 after the reading of a paper on the geological structure of the 

 district at the Geological Society by Mr. Clough, at which I wavS 

 present. I was guided to many of the localities mentioned in this 

 communication by information obtained from that paper. I regret 

 very much indeed that it has not yet been published. It deals with 

 the mutual relations of the rocks referred to in the present com- 

 munication, but so far as I can remember, it does not contain 

 detailed petrographical descriptions. 



The Quartz-felsites. 



I have observed these rocks in three localities in the porphyrite 

 region ; in the Ridlees Burn about 1-|- mile from its junction with 

 the Coquet ; in the Coquet itself about 1 mile above Shillmoor 

 Farm ; and in the Usway rather more than ^ mile from its junction 

 with the Coquet. In each case the rocks occur as dykes in the 

 porphyrites, and they are well exposed in the actual banks of the 

 several streams. There is no marked variation in the rocks from 

 the different localities, and they may therefore be described collec- 

 tively. In colour they vary from pale red to dull purple. In texture 

 they are not conspicuously jDorphyritic, the individual crystals rarely 

 exceeding a length of two mm. Examined with a hand lens the 

 most conspicuous mineral is seen to be biotite, which is evenly 

 scattered through the rock in the form of beautiful hexagonal tablets. 

 Crystals of felspar, usually of the same colour as the ground-mass, 

 but occasionally weathering white, may be recognized. Grains of 

 quartz are also seen to be present, but their importance as constituents 

 of the rock is not recognized until the thin sections are examined. 

 The matrix presents the usual felsitic character. 



Under the microscope the biotite is seen to be of a perfectly normal 

 character. Its boundaries are intact; a fact which shows that in the 

 case of this rock the magma exerted no corrosive action on the 

 mineral. The only inclusions are apatite and magnetite. Quartz is 

 the next mineral in importance. It usually occurs in irregular grains, 

 but occasionally in the form of more or less regular crystals. It 

 contains inlets and inclusions of the ground-mass, and sometimes the 

 form of the grain is as irregular as the island of Celebes. The 

 relation of the quartz to the gi'ound-mass is therefore seen to be 

 that which is so especially characteristic of the rhyolites. Mica occa- 

 sionally occurs as an inclusion in the quartz. The felspars are so 

 similar in appeai'ance to the ground- mass on account of the presence 

 of irregular and ill-defined brownish flecks and scales (ferrite : 

 Vogelsang) that they may sometimes be overlooked when examining 

 the rock by ordinary light. Under polarized light they become at 

 once apparent, and in the majority of cases may be identified as 

 orthoclase, a conclusion which is confirmed by Mr. Waller's analysis 

 (see p. 111). The ground-mass is perhaps the most interesting 

 portion of the rock so far as microscopic characters are concerned. 

 It ])resents several very interesting modifications ; but before pro- 

 ceeding to describe these, it seems advisable to make a few general 



