1864.] ADDRESS BY SIR CHARLES LYELL. 895 



light — I allude to the metamorphism of sedimentary rocl:s. Strata 

 of various ages, many of them once full of organic remains, have 

 been rendered partially or wholly crystalline. It is admitted on 

 all hands that heat has been instrumental in bringing about this 

 re-arrangement of particles, which, when the metamorphism has been 

 carried out to its fullest extent, obliterates all trace of the imbed- 

 ded 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 diflficult to explain in what 

 manner an amount of heat capable of so entirely changing the 

 molecular condition of sedimentary 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 min- 

 erals which enter most largely into the composition of the metanior- 

 phic rocks have not been formed by crystalli/.ing 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 process requiring a far less 

 intense degree of heat. Thermal springs, chai-ged with carbonic 

 acid and with hydro-fluoric acid (which last is often present in 

 small quantities), are powerful causes of decomposition and chemi- 

 cal reaction in rocks through which they percolate. If, therefore, 

 large bodies of hot water permeate mountain-masses at great 

 depths, they may in the course of ages superinduce in tliem a crys- 

 talline structure ; and in some cases strata in a lower position and 

 of older date may be comparatively unaltered, retaining their fossil 

 remains undefaced, while newer rocks are rendered metamorphic. 

 This may happen where the waters, after passing upwards for 

 thousands of feet, meet w^ith some obstruction, as in the case of 

 the Wheal-ClifFord spring, causing the same to be laterally diverted 

 so as to percolate the surrounding rocks. The eflScacy of such 

 hydro-thermal action has been admirably illustrated of late years 

 by the experiments and observations of Senarniont, Daubree, 

 Delesse, Scheerer, Sorby, Sterry Hunt, and others. 



The changes which Daubree has shown to have been produced 

 by the alkaline waters of Plombieres, in the Vosges, are more espe- 

 cially instructive. These thermal waters have a temperature of 

 160'^ F., and were conveyed by the Komans to baths through long 

 conduits or aqueducts. The foundations of some of their works 

 consisted of a bed of concrete made of lime, fragments of brick, and 

 sandstone. Through this and other masonry the hot waters have 



