39 



In these masses are veins of the predominating substances of 

 the enclosing rocks. 



The granite being the fundamental base, or the crystalline 

 shell of the globe, its thickness is not known. It has a polar 

 structure, and when the quantity of mica is considerable, granite 

 divides into parallel plates, or in other words becomes laminated, 

 and exhibits the meridional structure explained above. 



Gneiss is the laminated part of the granitic base, the same 

 identical mass ; the distinction being produced by the ingre- 

 dients tending to arrange themselves in parallel plates ; quartz 

 follows quartz, felspar follows felspar, and mica follows mica. 

 (See Plate VI.) 



As this crystalline arrangement and lamination of the funda- 

 mental base is produced by the continual circulating action of 

 the magnetic currents through the semifluid mass, the transi- 

 tion of the crystalline aggregation to the laminated structure is 

 necessarily insensible ; the action being like a simultaneous 

 growth of the granite northward. Hence a micaceous granite 

 produces micaceous gneiss, chloritic granite chloritic gneiss, &c. 



Schist, or Crystalline Slate. — This variety forms the termina- 

 tion of the granitic base, the branches and leaves, as it were, of 

 the great granitic trunks. The mica granite passes first into 

 gneiss, and the latter into mica schist by an almost imperceptible 

 gradation. This rock has been represented as stratified by a 

 mistake in confounding the stratified with the laminated struc- 

 ture. (See Plate XVI.) It is the final decomposition of the fel- 

 spar that distinguishes slate or schist from gneiss. (Plate VI.) 



It will therefore be observed that the primary crystalline, 

 from the granite to the schist, belongs to one formation, and is 

 essentially composed of the same minerals, variously modified by 

 the polar force, and passes by insensible gradation from the base 

 to the final slaty structure in a more or less vertical and meri- 

 dional direction; but subject to constant changes and disturb- 

 ances from local causes. 



These rocks are very extensively developed in South America, 

 and may be traced from Chili to the Caribbean sea. A section 

 was taken across the three Cordilleras, where the rocks were seen 

 cut by ravines upwards of 2000 feet deep, thus exhibiting na- 

 tural sections, and showing the nature of their transition vertically 



