192 MR. E. E. C0WPEE HEED ON THE GEOLOGY OE [May 1895, 



Where magnetite is very abundant the specific gravity naturally 

 rises ; thus in a rock from the neighbourhood of Manorowen [326] 

 it is 2-90. 



(5) Chemical Composition and Age. 



One of the tachyiytic rocks [101] from Garn Eawr has been 

 analysed with somewhat astonishing results (Analysis E below). 

 The microscopic characters of this rock have been described 

 on p. 183. 



E. F. 



Si0 59-15 Si0 6060 



A1 2 3 14-54 A1 2 3 17-99 



Fe 2 3 12-31 Fe,0 3 ] _.«„ 



K 9 6-56 FeO'j 7 6l 



Na 2 4-01 K o 380 



MgO 197 Na„0 238 



CaO trace MgO 137 



H 2 1-51 CaO 2-41 



H„0 3-20 



100-05 99-06 



Specific gravity 2 - 68 



There is evidence of phosphate of lime in (E). 



The analysis (E) has so abnormal an appearance that Mr. Tadman 

 took especial care with it and re-determined the silica, alumina, and 

 iron sesquioxide, but with the same results as before. The presence 

 of so much potash and soda in an intrusive rock with 59 per cent, 

 of silica suggests the lamprophyres, but the high percentage of iron 

 and the practical absence of lime make it impossible to regard this 

 as a special mode of consolidation of a lamprophyric magma. An 

 analysis by Mr. Collins of a minette from JNare Point, near Porthalla, 

 is here quoted for comparison (E). 1 



The differentiation of an igneous magma, owing to the researches 

 of Yogt, Wadsworth, Brogger, Teall, Harker, and others, is now 

 established, and it allows of so great a latitude of interpretation 

 that it is probably applicable to this case. 



In a rock previously mentioned [230] from Garn Eechan, adjoin- 

 ing Garn Eawr, a micropegmatitic groundmass has been described. 

 It seems probable, however, that it is of the nature of ' soda-micro- 

 cline ' as described in the rhomben-porphyry of Norway. The fact 

 that the Garn Eechan rock occurs close to the Garn Eawr rock, of 

 which the analysis has been given, has the same specific gravity 

 and many of the same microscopic characters, while it forms part 

 of the same intrusive mass, suggests a comparison with the analysis 



1 Some of the Carrock Fell variolites described by Harker (Geol. Mag. 1894, 

 p. 551) have a closely similar percentage of silica. For example, the Scurth 

 variolite, with 59'8 °/ together with almost identical microscopical characters. 

 But Eosenbusch's ' weiselbergite ' type of augite-porphyrite (' Mikroskop. 

 Physiogr. d. Mass. Gest.,' 2nd ed. 1887, p. 501) bears a close resemblance in 

 chemical composition, though with some important points of difference. An 

 analysis of this augite-porphyrite by Hetzer gives: Si0 2 58'97, A1 3 15-73 

 FeO 11-73, CaO 3-20, MgO 084, K,0 065, Na 2 5-43, and H 2 3-25". 



