THE CRYSTALLINE ROCKS NEAR ST. JOHN, N. B. 27 



the grains tlioiuselvcs, so that they cannot be distinguished 

 except by examining a thin section with the microscope. 

 The hmestone, probably once tine grained, or made up 

 of small fragments of the skeletons of low organisms, is 

 now crystalline, and shows (with a few exceptions) no 

 traces of its former condition. The gneiss was probably 

 once a sandy shale, but all of its original mineral grains 

 have disappeared, and we have in place of them a rock 

 composed of far larger grains of quartz, feldspar, mica 

 and hornblende, as densely packed as in a granite. The 

 original bedding of the rock is still quite plain, however, 

 as the sandier layers have been converted in a more 

 quartzose rock, and the more clayey ones into micaceous 

 bands. In Quebec and northern Ontario are vast areas 

 of gneissic rocks which arc much more metamorphosed 

 even, than those around St. John, and are extremely 

 massive and coarsely crystalline. They are everywhere 

 cut through by granite intrusions, and it is thought 

 doubtful whether they ever were ordinary sediments of 

 the modern type. But there can be little doubt about 

 the original nature of the gneisses near this city. 



Granite — Cutting through the Laurentian rocks 

 north of the city there are several granite intrusions. 

 The granite is fairly coarse-grained, showing that it 

 solidified at a considerable depth ; but it contains large 

 porphyritic crystals of orthoclase feldspar, which must 

 have formed at a still greater depth. In the mass of the 

 rock there is more plagioclase than orthoclase, hence it is 

 not properly a granite, but rather a quartz-diorite, except 

 in a few places where the orthoclase predominates. The 

 different areas of the rock differ considerably in appear- 

 ance, the wider ones being more coarsely crystalline, and 

 with more of the porphyritic crystals. But as all have 



