ON THE FORMATION OF CRYSTALLINE ROCKS. 277 



remoteness from the surface of radiation.* The superposition of 

 heavy detritus, as in some stratified formations, was often sufficient 

 to cause a notable increase of the heat of the masses beneath, after 

 their deposit, especially at epochs Avhen the increase of heat, in a 

 vertical line, was subject to a much more rapid law than now. 



This observation maybe added: that the bottom of the greater 

 part of the ocean does not now appear to exceed the temperature of 

 from three to four degrees centigrade, owing to the single fact that a 

 sedimentary deposit would be dried up in the temperate regions, and 

 that its surface would consequently gain several degrees of mean 

 temperature, all the points situated on the same vertical should in- 

 crease in temperature equally. f Thus the regular propagation of 

 the heat of the globe might have acted upon entire formations, and 

 produced upon them a gradual transformation which Elie de Beau- 

 mont characterized under the name of normal metamorphism. 



Setting aside the effects of this general, and, as it were, latent 

 cause, there are circumscribed places where the heat has come very 

 near the surface, especially in the suite of eruptive rocks. Hence 

 there are special centres around which internal heat has produced 

 accidental metamorphism or metamorphism of juxtaposition. Never- 

 theless there are very strong reasons for thinking that in both one 

 and the other case it is not heat alone that has acted. Even when 

 the temperature was sufficiently high to produce a softening of the 

 transformed rocks, which is most frequently altogether improbable, | 

 it would be insufficient to explain the diversity of effects which have 

 been ascertained. The following observations prove it: If heat alone 

 is the cause of the modifications observed in formations, the thick- 

 ness of which is often more than a thousand yards, how did its action 

 penetrate through such an extent? Whj^, at least, in accordance 

 with the known laws of the propagation of heat and the feeble con- 

 ductability of rocks, is it not of incomparably less energy in the re- 

 mote parts than in those near the surface by which it arrived? This, 

 however, is not the case, and the grandeur, as well as the uniformity 

 of effects produced in entire mountainous masses is a phenomenon of 

 the most striking significance. § Moreover, if, setting aside the rela- 

 tions of the general whole, we pass to facts of detail, we still find 

 in the manner of grouping of the minerals of the metamorphic rocks 

 a multitude of circumstances of association or of formation which 

 forbid our admitting for those minerals an origin due to heat alone. 

 To cite an example, I will mention the very frequent occurrence of 



<^ This remark is due to Mr. Babbage, {London and Edinburgh Philosophical Magazine, V, 213. ) 

 Sir John Herschel has made some experiments on the chemical reactions which the strata 

 have undergone owing to this ulterior elevation of temperature. — {Leonhard's Jarbuch, 1838, 

 p. 98 ; 1839, p. 347.) 



The bottom of the sea being at a low temperature, Hutton's idea of the heating of 

 the sedimentary deposits cannot be admitted in the terms in which he described it. 



f According to an observation of Sir John Herschel, noticed above. 



i Bischof and Durocher have insisted up )n arguments of this liind. 



§ Often, also, in metamorphism of juxtaposition, it is not always in the parts the nearest 

 to the contact of the eruptive roclis tliat tlie eifects have been most energetic. Durocher 

 has given several examples of this kind. 



