174 MINERALOGY AND LITHOLOGY. 



in New Hampshire the siHceous residues are the ones with which we 

 chiefly have to deal. It has been stated that such sedimentary beds can 

 become deeply buried, and that they may become plastic and enter into 

 a condition that is termed igneo-aqueous fusion under the combined in- 

 fluence of the weight of the sediments above them ; the lateral pressures, 

 the force of which is evinced in the elevation of our mountains ; the inte- 

 rior heat of the earth, and the water imprisoned in their mass, which in 

 such circumstances becomes a most powerful reagent. This theory, 

 which has been developed by the labors of several eminent geologists, 

 has been strengthened by microscopic examinations, first and most prom- 

 inent among which are those of Mr. Sorby, and it may be almost con- 

 sidered to be demonstrated. The features in these rocks that render the 

 theory so probable, will be pointed out in the proper places. 



Let us now suppose beds of great extent, such as may be derived from 

 basic rocks by one or by repeated disintegrations, to be subjected to the 

 influence of the agencies indicated. We may suppose, first, a wide varia- 

 tion in the composition of the sedimentary beds, resulting from differ- 

 ences in the agencies that had operated to decompose and re-deposit. 

 From this cause, much diversity in the derivative rocks would appear. 

 We may suppose a variability in the factors of heat and pressure, which 

 would modify the completeness of the recrystallization or metamorphism ; 

 hence some beds under powerful agencies would be entirely recrystal- 

 lized, while others more gently acted on would be only partially altered, 

 and would jaossess some of the original ingredients or features which 

 would indicate their relationship to the stratified deposits. We may 

 suppose, if no movements took place to disturb the beds while in this 

 plastic condition, that, though recrystallized into massive rocks, they 

 would maintain their position, and when solidified would still be con- 

 formable with the surrounding strata. On the other hand, if great move- 

 ments took place, such as must have accompanied the elevation of our 

 White Mountains, fissures might be produced, into which the plastic 

 rock might be forced, and the same identical rock might now present 

 itself as an eruptive mass. Under other circumstances, the overlying 

 strata might be crushed and mixed into the plastic mass ; and many odd 

 features in the arrangement and form of the masses might result from 

 circumstances. All those who are familiar with our acidic rocks know 



