METAMORPHIC EOCKS. 249 



almost at right angles with those of stratification, indicating 

 an action posterior to the alteration of the schist. ( 267 ) The 

 silicic acid which has penetrated into the mass causes it 

 to be traversed by veins of quartz, and transforms it in part 

 into whetstone and siliceous schist; the latter sometimes 

 containing carbon, and then perhaps capable of pro- 

 ducing galvanic phenomena. The most highly silicified 

 rocks of this kind are known as ribbon jasper, ( 26s ) a 

 material valuable in the arts, produced in the Oaral moun- 

 tains by the eruption and contact of augitic porphyry (as at 

 Orsk), of dioritic porphyry (as at Aufschkul), or of a 

 rounded mass of hypersthene rock (as at Bogoslowsk). 

 In the island of Elba (at Monte Serrato), according to Fried- 

 rich Hoffman, and in Tuscany, according to Alexandre 

 Brongniart, the ribbon jasper is formed by contact with 

 euphotide and serpentine. 



Sometimes (as observed by Gustav Rose and myself in 

 the Altai, within the fortress of Buchtarminsk), ( 269 ) the 

 contact and plutonic action of granite have rendered argil- 

 laceous schists granular, and transformed the rock into a mass 

 resembling granite itself, consisting of a mixture of feldspar 

 and mica, in which larger laminae of mica are found im- 

 bedded. ( 2 7o) We are told by Leopold von Buch, ( ' that all 

 the gneiss between the Icy Sea and the Gulf of Finland has 

 been produced by the metamorphic action of granite upon 

 the silurian strata. In the Alps near the St. Gothard, 

 calcareous marl has been similarly changed by the influence 

 of granite, first into mica slate, and subsequently into 

 gneiss/' ( 271 ) Similar phsenomena of gneiss and mica slate 

 formed under the influence of granite present themselves 

 in the oolitic group of the Tarantaise, ( 272 ) in which 



