ADDRESS. 



Ixvii 



Hot springs are, for the most part, charged with alkaline and other highly- 

 soluble substances, and, as a rule, are barren of the precious metals, gold, 

 silver, and copper, as weU as of tin, platinum, lead, and many others, a 

 slight trace of copper in the Bath waters being exceptional. Never- 

 theless there is a strong presumption that there exists some relation- 

 ship between the action of thermal waters and the fiUing of rents with 

 metallic ores. The component elements of these ores may, in the first 

 instance, rise from great depths in a state of sublimation or of solution 

 in intensely heated water, and may then be precipitated on the walls of a 

 fissure as soon as the ascending vapours or fluids begin to part with some of 

 their heat. Almost everything, save the alkaline metals, silica, and cer- 

 tain gases, may thus be left behind long before the spring reaches the earth's 

 surface. If this theory be adopted, it wiU foUow that the metalliferous por- 

 tion of a iissure, originally thousands of feet or fathoms deep, wiU never be 

 exposed in regions accessible to the miner until it has been upheaved by a long 

 series of convulsions, and until the higher parts of the same rent, together 

 with its contents and the rocks which it had traversed, have been removed 

 by aqueous denudation. Ages before such changes are accomplished ther- 

 mal and mineral springs will have ceased to act ; so that the want of identity 

 between the mineral ingredients of hot spiings and the contents of metal- 

 liferous veins, instead of militating against their intimate relationship, 

 is in favour of both being the complementary results of one and the same 

 natural operation. 



But there are other characters in the structure of the earth's crust more 

 mysterious in their nature than the phenomena of metalliferous veins, on 

 which the study of hot springs has thrown light — I allude to the metamor- 

 phism of sedimentary rocks. Strata of various ages, many of them once 

 full of organic remains, have been rendered partially or wholly crystal- 

 line. It is admitted on aU hands that heat has been instrumental in 

 bringing about this re-arrangement of particles, which, when the meta- 

 morphism has been carried out to its fullest extent, obliterates all trace 

 of the imbedded fossils. But as mountain-masses many miles in length and 

 breadth, and several thousands of feet in height, have undergone such 

 alteration, it has always been difficult to explain in what manner an amount 

 of heat capable of so entirely changing the molecular condition of sedimen- 

 tary masses could have come into play without utterly annihilating every 

 sign of stratification, as well as of organic structure. 



Various experiments have led to the conclusion that the minerals which 

 enter most largely into the composition of the metamorphic rocks have not 

 been formed by crystallizing from a state of fusion, or in the dry way, but 

 that they have been derived from liquid solutions, or in the wet way — a 

 jirocess requiring a far less intense degree of heat. Thermal springs, charged 

 with carbonic acid and with hydrofluoric acid (which last is often present in 

 smaU quantities), are powerful causes of decomposition and chemical reaction 

 in rocks through which they percolate. If, therefore, large bodies of hot water 

 permeate moimtain-masses at great depths, they may in the course of ages 

 superinduce in them a crystalline structure ; and in some cases strata in a 

 lower position and of older date may be comparatively unaltered, retaining 

 their fossU remains imdefaced, while newer rocks are rendered metamorphic. 

 This may happen where the waters, after passing upwards for thousands of 

 feet, meet with some obstruction, as in the case of the Wheal-Clifibrd spring, 

 causing the same to be laterally diverted so as to percolate the surrounding 

 rocks. The efficacy of such hydrothermal action has been admirablv iUus- 



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