XL] GRANITES AND GRANITIC VEIN-STONES. 207 



to the entire exclusion of carbonate of lime, by an admixture of 

 which, however, they graduate into the adjacent limestones. 

 These beds generally consist of pyroxene, sometimes nearly 

 pure, and at other times mingled with a magnesian mica, or 

 with quartz and orthoclase, often associated with hornblende, 

 serpentine, magnetite, sphene, and graphite. These pyroxenite 

 rocks are generally gneissoid or granitoid in structure, and 

 sometimes very coarse grained. They occasionally assume a 

 great thickness, and are then often interstratified with beds of 

 granitoid orthoclase-gneiss, into which the quartzo-feldspathic 

 pyroxenites pass by a gradual disappearance of the pyroxene. 

 The limestones often include serpentine, pyroxene, hornblende, 

 phlogopite, quartz, orthoclase, magnetite, and graphite ; so that 

 the same minerals are common to them and to the pyroxenic 

 strata, which may be looked upon as marking the transition 

 between the gneissic and the calcareous parts of the series. 

 These strata, marked by the predominance of calcareous and 

 magnesian silicates, appear, so far as known, to accompany 

 each of the limestone formations of the Laurentian, sometimes, 

 however, developed to a greater and sometimes to a less 

 extent. 



34. I have elsewhere called attention to the fact that the 

 highly micaceous schists and the argillites of the Green Moun- 

 tain and White Mountain series of rocks are, so far as known, 

 wanting in the Laurentian, and with them the characteristic 

 minerals of the latter series, staurolite, andalusite, and cyanite. 

 There are, however, beds of a highly micaceous rock in the 

 Laurentian which contain an unctuous magnesian mica with a 

 pyroxenic admixture ; these are very unlike the mica-schists 

 composed of a non-magnesian mica and quartz, with orthoclase, 

 which abound in the White Mountain rocks. These magnesian 

 beds belong to the calcareous horizons in the Laurentian 

 series, at which also occur the most numerous veins and the 

 principal minerals of economic value. It is also along these 

 horizons, marked by softer rocks, that the valleys and the arable 

 lands of the Laurentian areas are chiefly found, and for this 

 reason, also, the mineralogy of these parts is better known than 



