428 



Prof. J. Prestwich. 



[June 18, 



tion of a state of equilibrium ; nor is there any reason to suppose that 

 the rocks were at any time crushed in the complete manner accom- 

 plished experimentally by Mr. Mallet. Consequently much of the 

 heat developed would be dissipated during the long slow compression, 

 and the maximum effects estimated by him would not obtain in nature. 

 Still, as the experiments show that the weight at which the first 

 yielding of the rock takes place is not more than one- third of the 

 crushing weight, the thermal effects might still be considerable, pro- 

 vided the time the heat had to spread through the adjacent rocks 

 were not excessive. It is also certain that greater and more concen- 

 trated effects would result at the time of actual disruption and 

 faulting. The gigantic foldings of the rocks in the Alps, and the 

 magnitude of the faults there and in the Pyrenees, show how vastly 

 great the forces then in operation were ; and indicate how important 

 must have been the consequent rapid conversion of even a portion of 

 the work of these forces into heat. 



Amongst the objections that have been raised against the explana- 

 tion of some cases of alteration of sedimentary strata in mountain- 

 chains by ordinary normal metamorphism, is the one that unaltered 

 strata alternate with the altered strata. Sometimes an apparent 

 alternation is explained by inversion of the strata, or where that 

 does not exist, it may be due to the circumstance that differences of 

 mineral composition, or in the proportion of the water of imbibition, 

 have caused the metamorphism to affect different beds in different 

 degrees. On the theory of regional Tnetamorphism, in the sense I 

 would use it, another explanation suggests itself by the way in which 

 differences in the resistance of the rocks develop different quantities 

 of heat. Mr. Mallet has shown by experiments on the compressibility 

 of rocks at Holyhead, that, although certain slate-rocks were com- 

 pressed by precisely the same force before their elastic limits were 

 passed, yet owing to differences in their compressibility, the heat 

 developed in the rocks when released would render the quartz-rock 

 nearly three times as hot as the slate-rock. In this manner, there- 

 fore, it may be possible to account for a special and restricted meta- 

 morphism of some strata in mountain-chains, and for its frequently 

 localised occurrence. 



Further there are, as is well known, many strata which are not 

 usually metamorphic, but which are so when involved in mountain- 

 chains. Among many common examples of such metamorphism may 

 be instanced that of the lower cretaceous strata on the flanks of the 

 Pyrenees. They are there represented by dark schists and crystalline 

 limestones, while at a short distance from that range they consist of 

 marls and ordinary limestones. In the Alps, strata of middle Eocene 

 age are, at the Diablerets, converted into hard black slaty rocks, which 

 are purely local ; while the soft and earthy calcareous Nummulitic 



