140 Proposed Scientific search for [Feb. 



known regions, very valuable deposits of metals have been found, 

 and it is very certain that the identical rocks, in continuation from the 

 detritus of which gold is extracted, in Thibet, form also the site of 

 Darjeeling ; the whole being primitive, and chiefly of gneiss formation ; 

 it has generally been found, that when alluvial deposits containing gold 

 have been exhausted, all further search for that most valuable mineral 

 amidst the hills from which the alluviae had been originally washed 

 down, has been in vain ; and hence the conclusion usually come to, has 

 been that deposits of gold, in metalliferous rocks, are extremely super- 

 ficial ; and doubtless this conclusion is generally correct ; but not 

 universally applicable ; as is proven by the depth of the Veta-Grand 

 mines in Mexico, being 310 fathoms; and in Potosi, where silver pene- 

 trates a hill 18 miles in circumference, to the greatest depth explored. 



Though gneiss is the main formation of these hills, especially in the 

 lower elevations, doubtless many other masses of transition rocks exist 

 higher up. 



Throughout the whole plain of India, from Bengal to the bottom of 

 the deep wells in Jessulmere, and under the mica and hornblende 

 schist of Ajmeer, the same kind of very hard fme-grained blue granite 

 is found in solid and rolled masses ; and this granite must form the 

 real basis, and underlies all the other strata of the Himalayan moun- 

 tains. In all the mountains around Darjeeling, the gneiss is apparent ; 

 but from the appearance through a telescope, the more elevated naked 

 rocks seem to be solid granite and syenite. 



The granite, gneiss, and mica slate of the Andes are generally cover- 

 ed by transition rocks, porphyry green stone, amygdaloid, basalt and 

 other trap formations. In the province of Oaxaca granite and gneiss 

 are rich in silver and gold ; and also in many other regions ; but nei- 

 ther granite, gneiss or syenite, are by any means the richest in metals ; 

 on the contrary, throughout the Cordilieras of Mexico rich metallic 

 veins are found in a great variety of rocks, and the deposits, which 

 furnish almost all the silver imported from Vera Cruz, are primitive 

 slate grey wacke, and Alpine limestone. The richest of all silver 

 mines is in primitive clay slate, passing into talc slate. The silver 

 mines of Potosi in Buenos Ayres are contained in primitive clay slate, 

 and the richest of those in Peru, in Alpine limestone. Many of the 

 richest metalliferous veins are found to traverse Alpine and Jura lime- 



