Vol. 55.] INTO DIABASE AT SOEEL POINT (noETHERN JERSET). 433 



Sections, to be described later, show absorption of basic material on 

 the part of the granite and intense alteration in the diabase, so that 

 there is considerable probability in the assumption that this granite 

 is the same as the porphyritic rock so often mentioned in the fore- 

 going pages. 



In a thin section the orthoclase is seen to be seldom or never 

 idiomorphic, but to form rounded, irregular junctions with the larger 

 grains of quartz. It shows microperthitic structure constantly, and 

 not infrequently small plagioclases are embedded in it. The mica 

 forms irregular and ragged flakes, either brown or green, with 

 considerable pleochroism and absorption. The mineral appears far 

 from fresh, and some quantity of secondary material has formed 

 along the basal planes. Judging by other sections, there is a 

 possibility that this mica is derived from hornblende. A little 

 magnetite and some free plagioclase are present. One peculiarity 

 found in this and other rocks is the presence of an intergrowth of 

 felspar and (presumably) quartz ; the latter occurs in fine branching 

 threads. This structure, which has been often described, Lacroix 

 has figured under the name of quartz vermicule. 



As the specific gravity is slightly higher than that of the similar 

 rocks from the immediate neighbourhood, some of the more basic 

 minerals may possibly owe their origin to material absorbed from 

 the diabase. 



The typical brick-red granite of Sorel Point, apart from its still 

 greater poverty in the dark minerals, shows a different relation 

 between the quartz and felspar. As before, the rock is very rich 

 in quartz, which sometimes occurs in large rounded or rectangular 

 grains, but more characteristically in numerous close-set, rounded 

 or elongated drops, embedded in, or forming a network with, the 

 felspar (PI, XXIX, fig. 1). Adjacent drops often polarize together, 

 so that the structure is in some degree granophyric. The ortho- 

 clase shows plagioolase-intergrowth frequently, but not invariably. 

 Free plagioclase is far from scarce. There are a few grains of 

 magnetite, and a very occasional intensely pleochroic mica-flake- 

 A partial analysis gave the following results : — 



SiOg Silica 78-10 



ALO3 Alumina , 13'02 



K,0 Potash 4-52 



98-87 

 A small quantity of iron is included with the alumina. 



Deducting 1 from the alumina-percentage for the iron, the 

 mineralogical composition of the rock may be calculated approxi- 

 mately as follows : — 



Plagio- 

 clase. 



SiO^ Silica 19-71 



ALO, Alumina 6-45 



K2O Potash -30 



Na^OSoda 3-12 



29-58 

 Q.J.G.S. No. 219. 



Ortho- 







clase. 



Quartz. 



Total, 



19-71 



38-68 



78-10 



5-58 





12-03 



4-30 





4-60 



-30 





3-42 



29-89 



38-68 



98-15 

 2p 



