430 



Prof. J. Prestwich. 



[June 18, 



rocks are all on a small scale, and affect the adjacent rocks locally 

 only by contact metamorphism. It is probable that the anthracite of 

 the coal-field of South Wales is the result of similar regional meta- 

 morphism. 



In the case of certain contact metamorphisms produced by contact 

 with igneous rocks, we know that the changes were produced by 

 great heat, for many igneous rocks must have had a temperature of 

 3000° to 4000° F. or more ; while in the case of normal metamorphism 

 it is evident that the changes produced did not depend so much on 

 high temperature as on pressure and the presence of water ; and there 

 is reason to believe that a temperature of about 600° to 800° F. 

 would suffice to produce all or almost all the observed hydrothermal 

 effects. For although in many instances of normal metamorphism 

 new minerals are formed, the rocks are not fused, nor are the fossils 

 destroyed. In Brittany, black slates, which pass into schists, with 

 large crystals of Chiastolite, still show impressions of orthis, trilobites, 

 and other Silurian fossils.* Devonian strata in the Vosges pass into 

 a rock consisting of pyroxene, garnet, epidote, and other silicates of 

 this character, and yet retain impressions of corals.^ 



The degree of heat required, therefore, to produce changes similar 

 to those produced by normal metamorphism, under somewhat 

 analogous conditions of time, temperature, and moisture, is com- 

 paratively small ; and as affording some indication of this amount, 

 the alterations in the character of the coal which have taken place in 

 the above-named instances afford a convenient approximate test. 

 While it requires a red heat to convert coal into coke, its conversion 

 into anthracite is effected in presence of moisture at much lower 

 temperatures, and while contact with igneous rocks has produced the 

 former effects, contact with granite has only resulted in the latter. 

 M. Daubree has even converted wood, by exposure for some 

 time in water under pressure to a temperature of 300° C, into 

 an anthracite so hard as scarcely to be touched by steel, and so 

 infusible as to burn with extreme slowness even in the oxidising flame 

 of the blowpipe, while at the same time it has been rendered, like the 

 diamond, a non-conductor of electricity. This alteration in the coal- 

 beds indicates, therefore, the influence of a temperature sufficient to 

 produce effects similar to those pi-oduced by ordinary normal meta- 

 morphism, and consequently sufficient to raise very considerably the 

 temperature of a body of rocks such as form mountain-chains, though 

 insufficient to cause fusion. 



* Bobblaye : " Bull. Soc. Geol. de France," vol. x, p. 227. [Mr. John Postle- 

 thwaite, of Keswick, informs me that Graptolites and the fragment of a Trilobite 

 have recently been found in the nietaniorphic "Chiastolite" slate of Skiddaw. 

 —July, 1885.] 



t Daubree : " Greologie Experimentale," p. 141. 



