PLICATION. 



27 



This position seems to us to lie at the foundation of metamorphism. Yet as it is not 

 ;i<l\ anced in elementary treatises upon geology, as a principle capable of proof, we find it 

 necessary to bring out that proof; especially as the rocks of Vermont furnish some 

 remarkable evidence on this point, and, so far as we know, entirely new. 



We proceed on the supposition now generally admitted by geologists, that all the 

 stratified rocks were originally deposited from water and subsequently consolidated into 

 shales, sandstones, conglomerates, and fossiliferous, earthy and compact limestones. 

 Though the fragments have been cemented together by chemical agency, yet they still 

 retain, after consolidation, the evidence of the mechanical forces by which they were 

 crushed, comminuted and rounded. 



But if we start with such rocks, which manifest so much of mechanical and so little 

 of chemical agencies, and run back to the most highly crystalline, we shall find a series 

 of changes which indicate a plastic condition subsequent to consolidation ; a degree of 

 plasticity sufficient to allow of movements among the particles, and of course great chem- 

 ical changes, but not generally so complete a fusion as to obliterate all traces of original 

 deposition and lamination ; nor, in some cases, of the mechanical origin of the materials. 

 We present the following proofs of such plasticity: 



1. Their texture has been more or less changed from mechanical into crystalline. We find the process 

 indeed in all its stages, and this enables us to prove that it has actually taken place. 



2. The organic remains in these rocks have been sometimes elongated, or otherwise distorted, so as it 

 could have been done only while in a plastic state. More often these remains have disappeared entirely, 

 and a crystalline texture has supervened. This could have been done only by chemical agency, while the 

 materials were in a yielding state. For a change of crystallization can take place only where the particles 

 are free to obey the laws of molecular action for bringing them into new positions. 



3. The strata and folia of rocks now highly crystalline, have been subject to remarkable foldings and 

 distortions, such as only a plastic state of the materials will explain. Fig. 4 represents a bowlder of gneiss 

 and interstratified hornblende schist, obtained from the bed of Deerfield river at Shelburne Falls, in Massa- 

 chusetts, a few miles south of the Vermont line, but well representing many spots in the latter State. Few 

 geologists will doubt that mechanical pressure must have produced the beautiful curvatures of the layers. 



Fio. 4. 



Plicated Gneiss. 



