250 GEOLOGICAL DESCRIPTION OP THE EARTHS CRUST. 



belemnites are found in rocks which have already in great 

 measure assumed the character of mica slate, in the 

 schistose group of the western part of the island of Elba, 

 not far from Cape Calamita, and in the Fichtelgebirge 

 near Bai'reuth, between Lomitz and Markleiten. ( 273 ) 



I have already alluded to the jasper employed in the arts, 

 which the ancients could not obtain in large masses, ( 274 ) 

 and have described it as produced by the volcanic action 

 of augijic porphyry; there is also another material of 

 which ancient art made the noblest and most extensive 

 use, i. e. granular or saccharoidal marble, which is to be 

 regarded as a sedimentary rock altered by terrestrial heat 

 and the vicinity of erupted rocks. This assertion is 

 justified by a careful observation of the phenomena which 

 result from the contact of igneous rocks, and by the 

 remarkable experiments made by Sir James Hall on the 

 fusion of mineral substances. These experiments, made 

 more than half a century ago, together with the atten- 

 tive study of the phenomena 01 granitic veins, have 

 contributed in a very high degree to the recent pro- 

 gress of geological science. Sometimes the metamorphic 

 action of the erupted rock extends only to a very small 

 distance from the surface of contact, and produces a partial 

 transformation, or a sort of penumbra, as in the chalk of 

 Belfast in Ireland traversed by veins of basalt, and as in the 

 compact calcareous beds, partially inflected by the contact 

 of syenitic granite, near the bridge of Boseampo, and at the 

 cascade of Canzocoli in the Tyrol, brought into notice by 

 Count Marsari Pencati. ( 275 ) Another mode of transfor- 

 mation is, when the whole of the beds of compact limestone 

 become granular by the action of granite, syenite, or dioritic 

 porphyry. ( 276 ) 



