METAMORPHISM. 729 



as wide as the world, furnished abundantly with soda and magnesia, 

 and in smaller proportions with boracic acid and many other ingre- 

 dients. 



2. The effects on the same sedimentary rock have varied with the de- 

 gree of heat and pressure, and the amount of moisture. — Granite and 

 gneiss are examples of different results in consequence of difference in 

 heat and pressure. The differences between mica schist, mica slate, 

 hydromica slate, clay slate, appear to have arisen largely from the 

 differences of temperature attending metamorphism ; for, in going west, 

 in Berkshire County, Mass., the same formation, overlying the Stock- 

 bridge limestone, passes through these gradations. The stratum which 

 is chlorite rock in one part of a region of metamorphism is horn- 

 blende rock in another. 



An example of the effect of pressure is afforded by granular lime- 

 stone, or marble. It is usually the firmest, and least divided by frac- 

 tures or planes of bedding, and hence best for architectural purposes, 

 when its bedding is nearly vertical in position ; for, in that case, it has 

 been subjected to the greatest pressure, and the original bedding has 

 disappeared through a soldering together of the whole. 



3. The attending circumstances were favorable for the production 

 of subterranean heat. — The rocks, during a period of metamorphism, 

 are undergoing extensive displacements and foldings, profound fractur- 

 ings and faultings, as illustrated in the examples which have been de- 

 scribed. Metamorphic rocks are always displaced and folded rocks, 

 and never for any considerable distance horizontal. Where the fold- 

 ings are most numerous and abrupt, reducing the strata to a system 

 of parallel dips, by the pressing of fold upon fold, there, as remarked 

 by the Professors Rogers, the metamorphism is most complete. In 

 the case of mineral coal, the bitumen is more completely expelled, the 

 greater the disturbance of the strata ; and, in the metamorphic region 

 of Rhode Island, the coal has been changed even to graphite, by the 

 heat (p. 400). Now, if the transformation of this motion into heat can 

 produce fusion and volcanoes, as Mallet has explained, it is certainly 

 sufficient for the feebler work of metamorphism. It is then true, as 

 Wurtz was first to announce, that the heat of metamorphism was made, 

 in the very rocks that were altered, by the movements to which they 

 were subjected. 



The thermal springs of Virginia are regarded by the Professors 

 Rogers as owing their heat to the same cause which produced the con- 

 solidation and metamorphism in the Appalachian region ; and they 

 instance, as evidence of this, the fact that the localities where they 

 occur are generally situated over the axis of some fold in the Appa- 

 lachian strata. 



