64 DR. A. R. DWERRYHOTJSE ON INTRUSIVE ROCKS [Feb. I909, 



The marginal varieties. — Reference has already been made 

 to the changes in texture and composition which the granite under- 

 goes as the margin of the intrusion is approached, and it is now 

 proposed to describe some of the marginal varieties in greater 

 detail. 



The rock, as one would expect from the more rapid cooling which 

 must have prevailed along the margins of the intrusion, is much 

 finer in texture than in the more central parts. Another change, 

 one of composition, accompanies this, a change which at first sight 

 is not so easy of explanation. Samples of the normal Eskdale 

 Granite were found on analysis to yield from 71 - 86 to 76-43 per 

 cent, of silica, but specimens nearer the edges showed a progressive 

 increase in the silica-percentage, until at the extreme edge of the 

 granite in Hardrigg Gill (80) the rock contained 9646 per cent, 

 of silica. 



In every case in which I was able to examine the actual junction 

 of the granite with the Borrowdale rocks, these extremely acid 

 varieties were present, though the percentage of silica was usually 

 not so high as in the case of the Hardrigg-Gill specimen, being 

 more commonly in the neighbourhood of 90 per cent. 



Analyses op the Normal Granite and op the Marginal Specimen No. 80. 



Specimen 79. Specimen 80. 



Normal granite. Marginal variety. 



per cent. per cent. 



Si0 2 76-43 96-16 



A1,0 3 1356 1-31 



Fe" o 3 0-08 0-02 



Fet) 0-55 0-09 



MgO 0-54 003 



CaO 003 trace 



Na„0 4-19 0-56 



K„0 472 0-30 



HjO (combined) 0"76 051 



H 2 0(free) 0-06 008 



Totals 100-92 99-06 



Marginal varieties from Robin Gill (Mitredale). — An 

 exposure in the stream known as Robin Gill shows a fine-grained 

 granite of a pinkish colour (51) ; while at (53) close to the Upper 

 Waterfall the rock is of a felsitic texture, with small porphy- 

 ritic crystals of quartz and felspar, and a few dark red patches 

 apparently stained by haematite. At (54), nearer the junction, 

 it is generally similar, but with the porphyritic constituents less 

 conspicuous. 



Specimen 53. — A fine-grained, pink, felsitic rock, with 

 small porphyritic crystals of quartz and felspar. The ground-mass 

 is a fine-grained mosaic of quartz and felspar, with phenocrysts 

 .of quartz, orthoclase, perthite, and oligoclase, also of biotite which 

 has passed over into chlorite and hsematite. The rock moreover 



