50 Geology, as illustrated by Chemistry and Physics. 



the formation of rocks by igneous agency, which has been 

 partially mentioned already, and will partially follow. 



The doctrine of plutonic metamorphism certainly found a 

 feeble support in the transformation of glass into the so- 

 called Reaumur porcelain. It was imagined that an uncrys- 

 talline sedimentary rock might, by ignition and gradual 

 cooling, pass into a mixture of different crystalline minerals, 

 in the same way that glass packed in sand is transformed by 

 a red heat and gradual cooling into a crystalline mass. 



It was also attempted to explain the metamorphism of 

 rocks by the upward progression of the higher temperature 

 in the interior. If, for example, a deep marine basin is 

 gradually filled by sediment, the temperature of the original 

 bottom would be raised. That such is really the case, is 

 shewn by the increase of temperature in the sedimentary 

 rocks formed in this manner. If the temperature increases 

 at very great depths in the same proportions as in depths 

 which can be observed, then at a depth of 1 15,000 feet in 

 such a rock there would be a temperature of 1000° R. Such 

 a heat may indeed exceed that by which glass is converted 

 into Reaumur porcelain. At such a depth there would be 

 no difficulty in conceiving a transformation of an amorphous 

 sedimentary rock into a crystalline, if such a change actually 

 does take place at a red heat. 



Finally, it was imagined rocks might be so metamorphosed 

 by the vapour and gaseous exhalations rising from craters, 

 and in their neighbourhood, that crystalline rocks might be 

 formed from amorphous strata. 



There is no want of opportunities for the Plutonists to 

 study the changes which rocks undergo when exposed to the 

 continuous action of high temperatures. There are coal 

 strata which have been burning for centuries. Argillaceous 

 sandstone, clay, and slate clay, which form the roof of these 

 burning strata, have, during this long period, been exposed to 

 a very high temperature, and have been more or less altered. 

 But they shew no other alteration than that observed in 

 bricks after burning ; they appear fritted, half vitrified, and 

 scoriaceous. There has never been a crystalline mineral 

 met with in them, such as feldspar, which occurs in all crys- 



