43 2 La Plata : past il 



schist and the foliation of the gneiss were exactly 

 parallel. Eastward of the city there is much fine- 

 grained dark-coloured gneiss, almost assuming the 

 character of hornblende-slate, which alternates in thin 

 laminae with laminae of quartz, the whole mass being 

 transversely intersected by numerous large veins of 

 quartz : I particularly observed that these veins were 

 absolutely continuous with the alternating laminaB of 

 quartz. In this case and at Eat Island, the passage of 

 the gneiss into imperfect hornblendic or into chloritic 

 slate, seemed to be connected with the segregation of 

 the veins of quartz. 1 



The Mount, a hill believed to be 450 feet in height, 

 from which the place takes its name, is much the 

 highest land in this neighbourhood : it consists of horn- 

 blendic slate, which (except on the eastern and disturbed 

 base) has an east and west nearly vertical cleavage ; 

 the longer axis of the hill also ranges in this same line. 

 Near the summit the hornblende-slate gradually be- 

 comes more and more coarsely crystallised, and less 

 plainly laminated, until it passes into a heavy, sonorous 

 greenstone, with a slaty conchoidal fracture ; the 

 laminae on the north and south sides near the summit 

 dip inwards, as if this upper part had expanded or 

 bulged outwards. This greenstone must, I conceive, be 

 considered as metamorphosed hornblende-slate. The 

 Cerrito, the next highest, but much less elevated 

 point, is almost similarly composed. In the more 

 western parts of the province, besides gneiss, there is 

 quartz-rock, syenite, and granite ; and at Colla, I heard 

 of marble. 



1 Mr. Greenough (p. 78, ' Critical Examination,' &c.) observes that 

 quartz in mica-slate sometimes appears in beds and sometimes in 

 veins. Von Buck also, in his ' Travels in Norway ' (p. 236), remarks 

 on alternating laminae of quartz and hornblende-slate replacing mica- 

 schist. 



