518 Geology and Magnetism. 



of Scotland. The Irish crystalline series may be considered 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 crystalline slates are two inde- 

 pendent meridional series.* 



A variety of talcose, micaceous and chloritic schists may be seen 

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

 ing 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 complicated, by which 

 cause the phenomena of heaves, splits, veins, &c. are produced. 



Metalliferous rocks. — These are channels of rocks in which mi- 

 nerals are so abundantly disseminated that the whole masses are 

 worked like quarries. In the silver mines of Mariquita native 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 metal- 

 liferous channels are quarried for silver. In the same neighbour- 

 hood the argentiferous channels are very numerous. 



At Ibague, copper is found under similar circumstances in a 

 clay-slate formation ; also lead and iron pyrites, disseminated in 

 porphyritic rocks in the same locality. 



Gold is principally found as a superficial efflorescence on the face 

 of rocks and in cavities, seldom or never in a close grain or compact 



* This and other similar appearances referred to, are illustrated in the original 

 work; but except plate XXVII, exhibiting the polar currents as corresponding 

 with the direction and variation of the magnetic needle, we have not thought the 

 other illustrations essential to this notice. — Ed. 



