206 GRANITES AND GRANITIC VEIN-STONES. [XL 



a recent deposit in Iceland. The association of such hydrated 

 silicates with orthoclase, as already noticed ( 13), and as de- 

 scribed by Scheerer, where natrolite and orthoclase envelop 

 each other, showing their contemporaneous formation, with 

 many other facts of a similar kind, lead to the conjecture that 

 orthoclase, like beryl and quartz, and perhaps some other con- 

 stituents of granitic veins, may have crystallized in many cases 

 at temperatures much lower than those determined by Sorby, 

 and that the conditions of their production include a consider- 

 able range of temperature; a conclusion which is, however, 

 probably true to some extent of zeolites also. 



32. It is now proposed to consider the granitic vein-stones 

 found in Laurentian rocks. The stratified rocks of this ancient 

 gneissic series, as I have elsewhere pointed out, differ consider- 

 ably from those of the White Mountain series, which, with 

 their vein-stones, have been treated of in 16-23. 



The Laurentian series, the Lower Laurentian of Sir William 

 Logan, as studied by him in a region to the north of the Otta- 

 wa, the only area in which it has yet been examined in detail, 

 appears to consist of an alternation of conformable gneissic and 

 calcareous formations. The latter are three in number, each 

 from 1,000 to 2,000 feet or more in thickness, and separated 

 by still more considerable formations of gneiss and quartzite, a 

 mass of gneiss of great but unknown thickness forming the base. 

 (Geology of Canada, page 45.) The gneissic rocks of the 

 series are very firm and coherent, reddish or grayish in color, 

 often very coarse grained and granitoid, sometimes with but 

 obscure marks of stratification ; and frequently porphyritic from 

 the presence of large cleavable masses of reddish orthoclase, 

 occasionally with a white triclinic feldspar. They are often 

 hornblendic, and sometimes contain small quantities of dark 

 colored mica. A white granitoid gneiss, composed chiefly of 

 orthoclase and quartz, sometimes contains an abundance of red 

 iron-garnet. The latter mineral is often disseminated, or forms 

 subordinate beds in the quartzites of the series. 



33. With the crystalline limestones of the calcareous parts 

 of the series are often found strata made up of other minerals, 



