WATER. 



25 



potter's clay. Pseudo-trappcan and magnesian rocks are consumed, much like wood in a slow fire, and go 

 to form sulphate of magnesia and other products. Granite is rendered so soft that you may crush it 

 between your fingers as easily as bread unbaked. The feldspar appears to bo converted partly into 

 alum. In the meantime the bowlders and angular fragments brought down the ravine and river by 

 floods, are being cemented into a firm conglomerate, so that it is difficult to dislodge even a small pebble, 

 the pebble itself sometimes breaking before the cementation yields." Mr. Shepherd adds : "Themetamor- 

 phic action going on is at this moment effecting important changes in the structure and conformation of 

 the rocky strata. It is not stationary, but apparently moving slowly eastward in the Pluton Valley." 

 (Am. Journal Sci., Vol. XII., N. S. pp. 157, 158.) 



This spot seems to be an opening into the great laboratory of nature, where we get a 

 glimpse of the mighty works she has been carrying on in almost every part of the earth's 

 crust during the past geological ages. We have reason, however, to believe that the 

 action was more powerful in past times than at present, because the earth's crust was 

 thinner, and volcanic agency more common and energetic. Yet at the Pluton Geysers it 

 is energetic enough, and that too at a very moderate temperature, to melt and transform 

 all known rocks, unless it be pure quartz. 



But though a very high degree of heat does not seem to be necessary to most cases of metamorphism, 

 yet it is essential that there should be an increase of it in newly formed strata, that they may be changed : 

 and how may we suppose this to have been accomplished ? 



An eminent mathematician, Prof. Babbage, in 1834, proposed a theory to show how the surfaces of equal 

 temperature within the earth's crust might experience changes in a vertical direction. Thus suppose A B 

 (Fig. 2), to be the ocean's surface, and A D its bottom, rising into a continent above A. Let G F be a line 

 of equal temperature, say two miles below the ocean's bed, which line would be essentially parallel to the 

 surface. Let now the accumulations of sand, clay Fia 2 . . 



and gravel, which are constantly going on in the 

 ocean, raise its bottom to A C. This coating of 

 non-conducting materials would prevent the escape 

 of the heat, which rises from the heated interior, 

 and cause it to accumulate at a higher level, so 

 that the isothermal G F (line of equal temper- 

 ature) would rise to G E that is, as high as the bottom of the ocean had been filled. The increase of heat 

 might be sufficient to produce the metamorphisms which we find many of the stratified rocks to have 

 undergone. 



Another consideration deserves to be taken into the account. Different beds of rock require for their 

 fusion, or semifusion ,very different degrees of heat. Hence heat permeating upward through the successive 



beds ABODE (Fig. 3), might almost entirely melt some 

 (D), partially fuse others (B), obliterate the fossils in one (E), 

 and leave them more or less distinct in others (0 and A.) This is 

 exactly what we find in the earth, and what we might expect in 

 theory. 



Another fact may be explained on the same principles. If 

 we examine a rock formation over its whole horizontal surface, we 

 shall find sometimes that it has undergone very different degrees of 

 metamorphism in different parts. In one portion of the field we 

 find that the original rock has been transformed into gneiss, and in another into mica schist (as in the 

 Hoosic Mountain range in Massachusetts), in another part (as in Canada) but little altered, and containing 

 The statements above made show us how these different degrees of metamorphism 



FIG. 3. 



