33 



the structure of rocks beneath the surface, where chemical actions 

 are observed in operation, and various crystals constantly form- 

 ing, it is not possible to arrive at anything like a correct know- 

 ledge of the law which governs the superficies of our globe. 



In Auvergne the laminae of the gneiss and schist run nearly in 

 a direction from south to north, and dip to the west, that is, when 

 viewed on a great scale, for when examined more partially the 

 ordinary exceptions occur. In short, the continent of Europe 

 exhibits this uniformity of structure throughout. 



In the United States of America w T e find the same meridional 

 structure. In Virginia the gneiss, talcose and chlorite slates 

 run north and south, leaning from the perpendicular towards 

 the west. The Boston railway exhibits, by its numerous cuttings 

 in an east and west direction, the general verticality and meri- 

 dional structure of the schistose rocks for several miles in length. 



Along the north coast of South America, in the Carribbean 

 Sea and the West India Islands, the same structure prevails ; 

 and was minutely examined by the author, from east to west 

 across the three great branches of the Cordilleras, between the 

 latitudes of 4 and 6 degrees north. 



In an admirable essay published on this subject by Professor 

 Sedgwick, we find the following observations : — 



" In that variety of slate which is used for roofing, the struc- 

 ture of the rock has been so modified that the traces of its ori- 

 ginal deposition are quite obliterated ; and this remark does not 

 apply merely to single quarries, but sometimes to whole moun- 

 tains .... In the Welsh slate-rocks we see the cleavage planes 

 preserving an almost geometrical parallelism, while they pass 

 through contorted strata of hard slate, obviously of sedimentary 

 origin .... Crystalline forces have re-arranged whole mountain 

 masses of them, producing a beautiful crystalline cleavage, pass- 

 ing alike through all the strata .... And again, through all this 

 region, whatever be the contortions of the rocks, the planes of 

 cleavage pass on, generally without deviation, running in parallel 

 lines from one end to the other, and inclining at a great angle to 

 the west .... Without considering the crystalline flakes along 

 the planes of cleavage, which prove that crystalline action has 

 modified the whole mass, w T e may affirm that no retreat of parts, 

 no contraction in dimensions, in passing to a solid state, can 



