No. 6. J HARRINGTON — NOTES ON DYKES. 321 



V. Grenville, Lot 4, Range VI. The specimen from this 

 locality is from one of the newer dykes which, as already stated, 

 cut all the other rocks of the region. It consists of a dark grey 

 fine-grained base (sp. gr. 2.83) with occasional porphyritically 

 imbedded masses of hornblende, which are often accompanied by 

 a plagioclase feldspar. Calcite is also present in white cleavable- 

 masses, mostly filling cavities.* 



Microscopically this rock is very different from those already 

 described, but it requires much further study. The ground-mass 

 appears to consist of a mixture of plagioclase, biotite (very abun- 

 dant), and magnetite or titano-ferrite, with a good deal of a green 

 mineral which is probably an alteration product, and is not at 

 all dichroic. Here and there also there are almost colourless 

 crystals, which may prove to be olivine, very much cracked, and 

 often converted along the cracks into a pale green mineral. As 

 stated above, the rock is porphyritic, and a section cut across 

 one of the porphyritic masses shows it to consist of beautifully 

 striated plagioclase with embedded crystals of hornblende and a 

 little pyrite, while all these three minerals contain numerous 

 crystals of apatite, the largest cross sections of which measure 

 about 0.25 mm. Some of the cross sections are perfect hexa- 

 gons, but none of the crystals when viewed longitudinally show 

 perfect pyramidal terminations, but are generally rounded as seen in 

 figure 6 c of the accompanying plate. When examined with a high 

 power, most of them are seen to contain numerous cavities, which 

 in a few instances have been observed to contain bubbles, although 

 most of them appear to be empty. Almost without exception, 

 too, they contain black globular and sub-globular bodies (see 

 plate), which possibly take the place of the thin nail-like bodies 

 often found in the apatite of basalt. Some of the crystals contain 



*An analysis of this rock was published by Dr. Hunt in the 

 Geology of Canada and also in the American Journal of Science 

 (Second Series, Vol. XXXVIII., p. 174) from which the following is 

 extracted : " When in powder the rock effervesces freely in the cold 

 with dilute nitric acid, and the solution evolves red fumes on heating. 

 In this way there were dissolved, lime, equal to 8.70 per cent, of 

 carbonate, 0.50 of magnesia, and 6.50 of alumina and oxyd of iron 

 = 15.70 per cent. The residue dried at 212° F., equalled 83.80 per 

 cent. A portion of aluminous silicate had evidently been attacked 

 by the acid. The dried residue gave on analysis, silica 52.20, alum- 

 ina 18.50, peroxyd of iron, with some titanic acid, 10.00, lime 7.34 

 magnesia 4-17, potash 2.14, soda 2.41, volatile 2.50 = 99.26." 



