626 CHEMICAL DEPOSITS IN VEIJSTS. [Ch. XXXVIIL 



stones, and pebbles, at points where it departs most widely 

 from verticality. Hence at places, such as a, %. '717, the 

 miner complains that the ores are " nipped," or greatly 

 reduced in quantity, the space for their free deposition 

 having- been interfered with in consequence of the pre- 

 occupancy of the lode by earthy materials. "When lodes 

 are many fathoms wide, they are usually filled for the most 

 part with earthy matter, and fragments of rock, through 

 which the ores are much disseminated. The metallic sub- 

 stances frequently coat or encircle detached pieces of rock, 

 which our miners call " horses" or "riders." That we should find some 

 mineral veins which split into branches is also natural, for we observe the 

 same in regard to open fissures. 



Cherncal deioosits in veins. — If we now turn from the mechanical to the 

 chemical agencies which have been instrumental in the production of 

 mineral veins, it may be remarked that those parts of fissures which were 

 not choked up with the ruins of fractured rocks must always have been 

 filled with water ; and almost every vein has probably been the channel 

 by which hot springs, so common in countries of volcanos and earth- 

 quakes, have made their way to the surface. For we know that the 

 rents in which ores abound extend downwards to vast depths, where the 

 temperature of the interior of the earth is more elevated. We also 

 know that mineral veins are most metalliferous near the contact of plu- 

 tonic and stratified formationSj especially where the former sends veins 

 into the latter, a circumstance which indicates an original proximity of 

 veins at their inferior extremity to igneous and heated rocks. It is more- 

 over acknowledged that even those mineral and thermal springs, which, 

 in the present state of the globe, are far from volcanos, are nevertheless 

 observed to burst out along great lines of upheaval and dislocation of 

 rocks. '^ It is also ascertained that all the substances with which hot 

 springs are impregnated agree with those discharged in a gaseous form 

 volcanos. Many of these bodies occur as veinstones ; such as silex, 

 carbonate of lime, sulphur, fluor-spar, sulphate of barytes, magnesia, 

 oxide of iron, and others. I may add that, if veins have been filled 

 with gaseous emanations from masses of melted matter, slowly cooling in 

 the subterranean regions, the contraction of such masses as they pass 

 from a plastic to a solid state would, according to the experiments 6f 

 Deville on granite (a rock which may be taken as a standard), produce 

 a reduction in volume amounting to 10 per cent. The slow crystalliza- 

 tion, therefore, of such plutonic rocks supplies us with a force not only 

 capable of rending open the incumbent rocks by causing a failure of 

 support, but also of giving rise to faults whenever one portion of the 

 earth's crust subsides slowly while another contiguous to it happens to 

 rest on a different foundation, so as to remain unmoved. 



Although we ai'e led to infer, from the foregoing reasoning, that there 



* See Dr, Daubeny's Volcanos. 



