20 PICTURESQUE SKETCHES 



innumerable fractured edges in every direction, the 

 pounding action of moving water, especially if that 

 water was of a high temperature, might grind down 

 the exposed rock with extreme rapidity, and pro- 

 duce extensive deposits, rapidly filling up hollows 

 and depressions. But we can hardly suppose the ex- 

 istence of depressions so considerable as the thickness 

 of the gneiss and clay-slate would require ; and it is 

 far more reasonable to assume that a contraction of the 

 crust, the result of gradual cooling, produced a series 

 of wave-like motions in the earth's crust, alternately 

 elevating and depressing portions of the surface, and 

 sometimes producing a succession of elevations or de- 

 pressions on the same spot. However this may be, it 

 is certain that these old sedimentary rocks have been 

 not unfrequently altered so as to have become crys- 

 talline; and they are also very often cracked and 

 broken, the cracks being sometimes filled up with 

 rocks of a different kind, injected apparently in a 

 melted state, and sometimes with other materials, 

 also crystalline, and often containing a greater or less 

 proportion of metallic ore. 



Thus do these lowest sedimentary strata, whose 

 vast antiquity is in many cases unquestionable, but 

 which sometimes, like the granite itself, have been 

 elaborated at later periods, occupy a definite place 

 among the rocks of which the earth's crust is made 

 up. They mark, it would appear, a strange and 

 dark passage from that state which we have con- 

 sidered chaotic, to a condition of more regular and 

 quiet deposit ; they are, however, with reference to 

 fossiliferous rocks, azoic, or lifeless; and they are also 

 as a class almost as widely spread, and as distinctly 



