On Mineral Metamorphism. 367 



tamorphosis and the productions of new minerals by the 

 eflFect of heat on the individual cases. 



The distance to which the metamorphic influence of heat 

 extends, or the thickness of the metamorphosed portions of 

 the rock which separates the original rock from the eruptive 

 mass, is very unequal. The influence of the trap veins or 

 dikes on the adjoining rock, and even of the great overlaying 

 trap upon the subjacent rock, seldom extends beyond a few 

 fathoms. On the Krazzenberg, near Cassel, the shell-lime- 

 stone (muschelhalk) has been altered to the extent of one 

 foot from a basaltic vein passing through it; at Hartford, in 

 Connecticut, variegated sandstone has been altered to the 

 extent of five feet under a covering of dolerite ; the change of 

 the brown coal at the Meissner extends, on an average, to 

 eight feet under the basalt; that of chalk, in Ireland, to 

 ten feet. The thickness of metamorphic coal sandstone, pro- 

 duced by a trap-dike in Ireland, extends even as far as forty 

 feet ; that of the metamorphic clay-slate, in Anglesea, ex- 

 tends to fifty feet ; that of the metamorphic or altered coal 

 at Blythe, in Northumberland, to ninety feet. Granite, sye- 

 nite, and analogous rocks, appear to have exercised a much 

 more widely-extended influence, whether it be that a longer- 

 continued calorific radiation took place through these, as 

 also their crystalline condition indicates, or that their effect 

 on the adjoining rock has been altogether different from that 

 of fiery molten masses. The metamorphosis of the lime- 

 stone and of the clay-slate, near Christiania, extends to up- 

 wards of 1000 feet from the granite, — according to Durocher, 

 even as far as 1000 yards or metres ; that of the limestone 

 of the Pyrenees, in S. Paul de Fenonillet, to at least 900 

 feet ; the thickness of the white marble of Predazzo, on Pal- 

 lerabbiose, and on Monzoni, or the gallestro slate and red 

 jasper in Tuscany and in Elba, will be scarcely less ; the 

 influence of granite upon the clay-slate of Brittany extends, 

 according to Durocher, to the distance of from two to three 

 killometres.* 



* A killometer is equal to 1093J yarils. 



