AGENTS OF CHANGE. 23 



We do not feel justified in dismissing this subject with such general views. For so intimately is meta- 

 morphism connected with the rocks of Vermont, and so much is it illustrated by them, that they cannot be 

 understood without an accurate knowledge of metamorphism ; and science demands that we should describe 

 whatever facts have fallen under our noticel that will throw any light upon so difficult a subject. It 

 is one of those points in geology about which its ablest cultivators still entertain considerable diversity of 

 opinion. Though we scarcely had any opinions concerning it when we began the examination of Vermont, 

 we have been gradually compelled, by the facts, to adopt in the main the views so ably put forth and 

 defended by such writers as Bischof, Lyell, Hunt, Jukes and others. Some of these views are so recent 

 that they have not found their way into many of our elementary treatises on geology, and we judge it desir- 

 able that we should state the general principles of metamorphism, especially as the rocks of Vermont 

 furnish us with some facts that, so far as we know, have not been before brought out, except in a little work 

 on Elementary Geology which we have recently published. New and important facts have come under 

 our notice within a few days (Sept. 1860), which we shall combine with the statements in that work, whose 

 language we shall employ where we cannot alter it for the better. 



We regret the necessity of using many technical phrases, and of presuming upon a 

 good deal of acquaintance with geology, in the readers of this part of our Report. Any 

 who choose, can pass it over till they have read the details of Vermont geology in the 

 subsequent pages, when this part will be more intelligible. 



The metamorplmm of a rock in its widest sense, is its transformation from one kind 

 into another. Consequently it takes place after the original formation of the rock. 



The 1erm " Metamorphic Rocks" has been used by Sir Charles Lyell and others in a much more limited 

 sense, to designate a class of rocks (mica schist, talcose schist, gneiss, &c.), that have been so transformed 

 as to have become crystalline, and to have lost for the most part their original structure. But this is 

 only one case of metamorphism. Prof. John Phillips, also, limits metamorphism to rocks that have been 

 altered by heat ; whereas it appears that water and other agents have played quite as important a part in 

 the change as heat. 



AGENTS OF METAMORPHISM. 



Heat is a most important agency, and a certain degree of it is probably indispensable ; 

 and yet other agencies effect important transformations of rocks at a temperature not 

 above that of the atmosphere generally. Yet the most striking examples of metamor- 

 phism were first observed in the vicinity of trap dikes, where chalk was changed into 

 crystalline limestone, clay into clay slate and mica schist, and fossils were obliterated. 

 Hence it was natural to suppose that wherever such effects were seen, dry heat had been the 

 cause, since the trap dikes were regarded as having been once in a melted state. But it 

 has been found that other agencies might be concerned even in the case of dikes. 



Water is one of these agents. It acts in two ways ; first in connection with heat, 

 secondly by its power of dissolving all rocks, and as the carrier of chemical re-agents to 

 aid in the work. There is a third mode in which it sometimes prepares the way for 

 chemical metamorphic action, viz. by freezing in the minute fissures of rocks, and thus 

 opening them to the influence of decomposing agencies. 



Professors W. B. & R. E. Rogers subjected 48 species of silicious minerals, rocks, glass, porcelain, &c., 

 to the action of pure water, and of water charged with carbonic acid. The minerals and rocks were such 

 as feldspar, hornblende, augite, schorl, mica, talc, chlorite, serpentine, epidote, dolomite, chalcedony, obsidian, 



