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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



or Triassic strata in this part of the world are referable to three 

 distinct epochs : — 



1. Their upheaval by the porphyries of Oolitic age. 



2. The protrusion of the still later dioritic rocks. 



3. The eruptions of the volcanic rocks, properly so called. 



All these have been already treated of more or less in detail ; and 

 in the sections accompanying this memoir (PI. II.) these occurrences 

 are in themselves sufficiently obvious to require no further expla- 

 nation. 



8. Carboniferous Formation. — The rocks of Carboniferous age met 

 with in Bolivia, to the west of the high Andes, appear, at intervals, 

 as small, elongated, basin-shaped deposits, the longer axis of which 

 is more or less north-west and south-east ; these basins are situated 

 in the midst of the great western diluvial plateau, showing them- 

 selves to the north at the Lake of Titicaca, and, further south, in 

 the provinces of Arque and Oruro. 



The portion of this formation examined by me is shown in the 

 accompanying Map (PI. I.) and Section No. 1 (PI. II.), where it 

 forms the Isthmus of Copacabana in the Lake of Titicaca, the pro- 

 jecting headland on the other side of the Straits of Tiquina, and the 

 islands in the lake itself. It is of very small extent when compared 

 to the immense areas occupied by the other sedimentary formations 

 here treated of ; but it is everywhere highly fossiliferous, and pre- 

 sents a fauna which leaves no doubt as to its geological age : the 

 lowest elevation of any part of it visible is about 12,500 feet ; and it 

 ascends from that height up to fully 14,000 or 15,000 feet above the 

 level of the sea. 



The unfortunate circumstance that war was declared between the 

 republics of Peru and Bolivia when I was in this part of the country 

 prevented me making anything but a most superficial examination. 

 As this isthmus is divided between the two nations by such a ser- 

 pentine line of frontier that in a day's journey in a straight line the 

 traveller enters and leaves the territory of one or the other of these 

 republics no less than seven times, and both lines of frontiers were 

 occupied by the respective hostile armies, a geologist was placed in 

 a very suspicious and uncomfortable position — as I had reason to 

 experience. I was therefore glad to get over the ground as quickly 



analyses of the copper-sands from San Bartolo, showing the composition of the 

 ores found there in the cupriferous sandstone : — 



1. 2. 



Copper 57 3 34-3 



Insoluble matter 1 or . cn o 



Silica, &c. } 3o ' 4 502 



Alumina, Iron, &.c. \ „ „ 



Soluble in acids J ** ra 



Carbonate of lime 14 6 9 



98-5 98-7 



the deficiency in the above analyses being due to a portion of the copper being in 

 a state of oxidation in the ore: thus, in No. 1, 99 per cent, of copper were dis- 

 solved in dilute hvdrochloric acid. 



