474 



ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



We may now proceed to consider what effects, according to 

 this theory, would be produced on the earth's crust as the same 

 was constituted after the slate rocks above mentioned, and even. 

 the so-called greywacke series had been deposited. The slates 

 and sandstones of the latter formation are the oldest rocks which 

 thoroughly resemble, in their lithological characters, the sedimen- 

 tary deposits of later periods ; wherefore we may suppose that at 

 the same time they were formed, the temperature of the earth's 

 surface and the agencies at work upon it somewhat approxi- 

 mated to those of the present day. The portion of the earth's crust 

 least likely to be affected by the subsidences consequent upon the 

 contraction of the globe, may reasonably be supposed to have 

 been the thickest part, that part where vertical strata of gneiss and 

 rocks allied to it, extended deep down into the earth's crust. The 

 part most liable to be fractured and raised into folds, would mo3t 

 probably be the thinnest, or that part where horizontal or but 

 slightly inclined gUeiss strata, had been conformably overlaid by 

 micaceous, argillaceous, chloritic and quartzose slates. If we at- 

 tempt to speculate as to whst might be the first consequences of 

 the contraction upon these latter rocks, we would naturally sup- 

 pose that after a fissure had once been formed, the strata border- 

 ing on it would rise in a manner sketched in the subjoined figure, 

 c b a b c 



a. gneiss, b. mica schist, c. clay slate. 

 And in reality not a few of the so-called Primitive Slate districts 

 possess au architecture closely analogous to the above ideal sec- 

 tion. This is especially the case in the Alps of Salzburg and 

 Upper Oarinthia. In this part of the central Alps, according to 

 Credner, a mass of granitic gneiss, drawn out from east to vrest, 

 forms the centre. On the north as well as on the south side of 

 this mass crystalline slates overlie it. On the north side the dip 

 is at a high angle to the north, and on the south side the highly 

 inclined strata dip to the south. These crystalline slates are 

 divisible into three groups, the lowest consisting of common and 

 calcareous mica-slate, the middle group of chlorite and talc slates, 

 and the upper group of common and calcareous clay-slate. More- 

 over the structure of the inetamorphic rocks of eastern North 

 -America, and also of the slate districts north of the Mjo'sen in 



