EFFECTS OF LATERAL PRESSURE. 173 



force requisite to crush a rock will melt one-tenth or two-tenths, or 

 any greater or less fraction of it, further contraction in deeper layers 

 must develop additional heat in the crushed layers, and hence we 

 may admit the sufficiency of the means to develop the heat which 

 is necessary. But if these views are to be substantiated, they ought 

 to receive elucidation from the structure of the rock in districts 

 which exhibit effects of pressure ; and in the ways in which folded 

 rocks have been modified, should be found proofs that heat augments 

 towards the centre of a region which has been uplifted by lateral 

 pressure. 



Where the rocks have been greatly compressed, as in Wales, the 

 Lake district, or Scotland, the old clays are altered into slates. Their 

 chemical composition is not changed, but they have lost their 

 elasticity as much as clay which has been burned into brick. And 

 Mr. Sorby has demonstrated, by sections of the rocks examined under 

 the microscope, that this change is due to an incipient crystallisation 

 by which a large part of the rock matter has become transformed into 

 a mineral, arranged in minute plates which have the characters of 

 mica or chlorite. 1 The immense compression and contortion which 

 these rocks have undergone is well known, but nowhere perhaps 

 better seen than in the section near Llanbabo in Anglesey. 2 



Whenever mountain- masses exist which show crystalline rocks 

 entering into their structure, those rocks always occupy the central 

 axis of upheaval and have the slates on their flanks. The outer part of 

 the crystalline mass consists of schists, which show a foliated structure 

 with the layers made up of crystals dovetailed and densely packed ; and 

 these rocks often pass by insensible gradations into slates on the outer 

 limit, and into granitic rocks towards the central axis ; while large bosses 

 of indubitable granite are often exposed in the centre of the chain. 

 Here is a sequence of changes, increasing in intensity, giving evidence, 

 from more perfect crystalline texture, of augmenting temperature, as 

 we approach the position in which the lateral pressures most perfectly 

 antagonise each other, and become most intense. So that since the 

 bulk of masses of granite and schists often bears some proportion to 

 the evident elevation of the region in which they occur, as in the 

 Pyrenees and Alps, though the present heights are often diminished 

 by denudation we may recognise in mountain structure and mineral 

 condition, exactly such phenomena as ought theoretically to exist, if 

 the heat which altered the rocks were produced locally by compres- 

 sions. And under no other explanation can we account for the 

 observed physical structure of mountain-chains. 



Temperature and Pressure involved in Rock Construction. 

 There is no means of estimating the extreme slowness with which 

 these changes have been brought about, except such as are suggested 

 by the changes of level in land which have been observed to be now 

 in progress ; and although high temperatures may be necessary to 

 approximate to similar conditions in our experiments, it is probable 



1 Sorby, Quart. Jour. Geol. Soc., vol. xxxvi. Address, p. 73. 

 - Ramsay, Mem. Geol. Surv., vol. iii. p. 246, fig. ico, new ed. 



