PHYSICAL HISTORY OF NEW HAMPSHIRE. 521 



heterogeneous deposits possessed peculiar chemical characters, they still 

 remain. There has been chemical action in the midst of the particles of 

 each stratum. The atoms have been perfectly free to move about and 

 enter into new combinations. And it is easy to understand what condi- 

 tions may insure these results. If the rocks are charged with hot water 

 or steam, they will assume a considerable plasticity, and the atoms will 

 be free to move in all directions, perhaps confined by the walls of particu- 

 lar strata. Other conditions favor the chemical reactions. Great pres- 

 sure gives energy to the action, and there is an abundance of time allowed 

 for the completion of the work. In a furnace the fire is removed, and 

 the minerals have very little time to crystallize out. But the metamor- 

 phic action may continue for hundreds or thousands of years without 

 diminution. It is this long continued constant agency which accom- 

 plishes in the end as much as a more thorough melting in a short time. 



The conditions favorable for the metamorphism of rocks are developed 

 during periods of elevation, i. The motion of shoving along the strata 

 is converted into heat. 2. When this force has crushed rocks, an 

 immense amount of heat has been liberated, enough to melt entirely the 

 mashed material. 3. Elevating forces are connected with displays of 

 subterranean heat. When strata are broken, immensely large fissures 

 are made, which extend down to igneous masses below the crust, not 

 necessarily a melted interior, but large reservoirs, comparable with oceans 

 for size. Compression may bring portions of this igneous material to the 

 surface through the fault, or at least send up strong thermal influences. 

 All these combined are sufficient to produce metamorphic changes. 



The facts of plication and disturbance are everywhere evident in the 

 Atlantic rocks, so that, whether we fully understand the process or not, 

 it is clear that the formation has been subjected to influences capable of 

 rendering them plastic, and thus of allowing chemical changes. 



As to the character of the original strata, it is likely that they were all 

 derived from the older porphyritic strata by the action of currents of 

 water of different velocities. The ordinary gneisses correspond with the 

 average composition of the older rock, and may have resulted from their 

 disintegration, the newer strata being argillaceous sandstones. The 

 ferruginous rock is fully four fifths silica, and its origin may be ascribed 

 to those currents which were fitted to transport finely divided silica. As 

 VOL. i. 68 



