516 Geology and Magnetism. 



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 mica- 

 ceous gneiss, chloritic granite chloritie 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 con- 

 founding the stratified with the laminated structure. (See Plate 

 XVI.) It is the final decomposition of the felspar 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 essential- 

 ly 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 meridional direction ; 

 but subject to constant changes and disturbances 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 natural 

 sections, and showing the nature of their transition vertically 

 as well as horizontally ; the minute, and very laborious investiga- 

 tion of which is the foundation of the present observations. The 

 crystalline series in Europe falls into insignificance when compared 

 with those of America, and it is in such extensive areas that the 

 real character of the crystalline base can be clearly ascertained. 



Besides the regular transition of the crystalline base into slate, 

 there are also veins formed, interlaminated in the mass. Should 

 the base contain a large proportion of magnesia and talc, veins of 

 serpentine rocks will be formed ; should the granite predominate 

 in silica, quartz veins will be found very abundant. 



* We have been unable to give the plates referred to, but the structure repre- 

 sented is so well described by Mr. Hopkins, that the reader can be at little 

 loss. — Ed. 



