41 



few meridional channels intersecting Cornwall, presenting the 

 usual phenomena of the vertical polar cleavage, and cleaving 

 the superincumbent beds, which may be seen at the United Hills, 

 St. Agnes, and various other parts of the northern coast. The 

 interlamination of the granite, porphyry, and slate, and the com- 

 mon northward transition, may be seen at Dolcoath, and along 

 the whole range of the Carn Brea granite on the north side. 



The gradation of granite into gneiss, and gneiss into mica and 

 clay slates, may be seen in Wicklow, and also in the western 

 part of Scotland. The Irish crystalline series may be consi- 

 dered as the southern extremity of those of Scotland. It has 

 been conjectured that the slates of the western part of North 

 Wales formed the eastern edges of those seen in Wicklow ; this 

 idea originated from the mistaken notion that primary schist 

 were sedimentary beds. The Wicklow and North Wales cry- 

 stalline slates are two independent meridional series. (Plate IX.) 



A variety of talcose, micaceous and chloritic schists may be 

 seen near Holyhead and on the south-west coast of Carnarvon, 

 possessing that fibrous structure and silky striated and shining 

 lustre in the planes of the laminae or meridional cleavage, so 

 peculiar to all the crystalline rocks. 



To enumerate the localities of the primary series would be an 

 endless task ; therefore we must refer to other works, and confine 

 ourselves to the mode of their formation and general structure. 

 As the action in the primary base is constant, like a series of 

 channels growing northward, with their pores and cleavages full 

 of mineral solutions, subject to variable tensions, fractures, &c, 

 the structure of the compound becomes occasionally very com- 

 plicated, by which cause the phenomena of heaves, splits, veins, 

 &c. are produced. 



Metalliferous rocks. — These are channels of rocks in which 

 minerals are so abundantly disseminated that the whole masses 

 are worked like quarries. In the silver mines of Mariquita na- 

 tive silver is commonly found in flakes, like mica in the laminae 

 of the schist, in channels of ground of about twenty-four feet wide, 

 of course in the meridian, like the formation of the rock itself; and 

 these metalliferous channels are quarried for silver. In the same 

 neighbourhood the argentiferous channels are very numerous. 



At Ibague, copper is found under similar circumstances in a 



