§ 30. METAMORPHISM OF ROCKS. §53 



ferous deposits, and surrounded at their base by Triassic 

 strata, forms the range of hills known as Charnwood Forest, 

 in Leicestershire. The highest ridge, Bardon Hill, is crested 

 with bare and rugged masses of syenite ; and in various 

 quarries opened at the base of the hills, interesting sections 

 are exposed of the relative positions of the crystalline 

 masses and the sedimentary strata. This isolated cluster 

 of primary rocks is within a hundred miles of the tertiary 

 deposits of the south-east of England ; and at a less dis- 

 tance from the metropolis than any other plutonic region.* 



30. Metamorphish of Rocks. — The transition from 

 granite to porphyritic trachyte, passes through infinite gra- 

 dations, but all the modifications appear to be referable to the 

 degree of incandescence of the materials, the circumstances 

 under which they were ejected, and their slow or rapid 

 refrigeration. An instructive example of the passage of 

 granite into basalt, described by Dr. Hibbert, will illustrate 

 these remarks. In one of the Shetland Isles, a bed of basalt, 

 extending for many miles, is seen in contact with granite, 

 At a little distance from the junction of the rocks, the 

 basalt contains minute particles of quartz, and these become 

 larger and more distinct as they approach the granite : 

 hornblende, felspar, and greenstone (the latter is a homo- 

 geneous admixture of hornblende and felspar) next appear ; 

 still nearer, the rock consists of felspar, quartz, and horn- 

 blende : and at the line of junction, felspar and quartz form 

 a mass, which requires but the presence of mica to be 

 identical with the granite in which it is insensibly lost, j 



Limestone in contact wath schist frequently assumes a 

 crystalline structure, as if the same agency which had con- 

 verted the clay into schist, had extended its influence to the 

 overlying calcareous beds. In the Isle of Man, interesting 



* See Excursion to Charnwood Forest : Medals of Creation, vol. ii. 

 p. 974. 



f Edinburgh Journal of Science. 



