520 Geology and Magnetism. 



generally filled with quartz and auriferous pyrites, being the princi- 

 pal metalliferous solvents of this chain. 



The central Cordillera is very schistose, thus possessing great 

 tenacity, and capable of being elongated ; consequently the trans- 

 verse fractures are but few, and confined to subordinate granular 

 channels : the longitudinal and diagonal splits are of ordinary 

 occurrence, the sides are commonly grooved and highly polished by 

 friction caused by the meridional movements of the parallel masses. 

 The hornblendic, calcareous and talco-magnesian varieties are found 

 extremely active in the above series. The eastern Cordillera is 

 very quartzose, therefore the east and west fractures are very 

 numerous, and are intersected by a few polar splits, with striated 

 and polished sides. In Mexico the porphyritic variety predominates, 

 and the meridional splits are consequently intersected by a great 

 number of fractures. In Cuba and the other large islands of the 

 West Indies the same phenomena are observed. In the southern 

 departments of the United States, and especially at Virginia, the 

 polar splits predominate. These splits have cleaved the coal beds 

 of Blackheath into longitudinal fragments, causing great disorder 

 in their position. In Cornwall they are very numerous ; the splits 

 are known by the names otfluccan and cross courses, and the trans- 

 verse fractures are called lodes. It is in the fractures that the 

 mineral wealth of this country is found. 



The great polar splits of Cornwall and Devon extend across the 

 Bristol Channel to Wales, and have cut the coal fields and all the 

 sedimentary rocks of the province into meridional strips. The 

 same kinds of splits and fractures are seen throughout England, 

 Scotland and Ireland, and have broken the great sedimentary beds 

 into various fragments of a somewhat rhomboidal form, according 

 to the oblique angles of the splits. 



On the continent of Europe similar series have been observed, 

 especially in Tangiers, Spain, France, Germany, Hungary and 

 Sweden ; we need not detail them, but beg reference to works de- 

 scribing each district. 



These splits and fractures, and their being in continual motion 

 by the constant action of the polar force, produce great disorder 

 in the general structure, and cause dislocations in the order of 



