40 PEBBLES CHEMICALLY CHANGED. 



quartz, having often more or less of a vitreous aspect. In fact it is nearly pure silcx, and 

 it is that form of silex which is absolutely insoluble in anything but hydrofluoric acid ; nor 

 can we suppose the presence of any heat high enough to melt it, without completely des- 

 troying the forms of the pebbles. Yet the evidence that they have been in a plastic 

 state, so as to be moulded by pressure, is too decided to be resisted. How then have they 

 been softened? 



Let us recur to the conglomerate at Newport. Most of the pebbles there are not pure 

 quartz, but rather silicates ; that is, silex united with bases, such as alumina, magnesia, 

 lime and iron. Now the silicates are soluble by water containing alkalies ; and such 

 quartz as the Vermont pebbles is the residuum of their decomposition ; that is, the bases 

 are abstracted to form other compounds, and the quartz is left. This is the most probable 

 theory of the origin of quartz rock generally. Even if we suppose it produced from sand- 

 stone, we know of no other way in which it could have been formed, for nearly all the 

 sandstones are silicates. 



But suppose the silicates in the form of pebbles to be permeated by water containing 

 alkalies could their bases be abstracted without entirely destroying the form of the peb- 

 bles ? We do not see why this could not be done, if kept in such a state by the water 

 that they could obey the laws of chemical affinity and form new compounds, and leave 

 the quartz behind. The chief effect upon the pebble would be to reduce its bulk, though 

 we do find sometimes the cement so strongly adhering to the pebble that it can hardly be 

 separated so as to leave a smooth surface. The bases we think would be mostly used in 

 forming the micaceous or talcose cement, and if there was a good deal of pressure upon 

 the whole rock, as we may reasonably suppose if the pebbles did shrink some, the cement 

 would increase and the whole would be kept compacted together. 



To sustain the position that the mineral constitution of pebbles in conglomerates is some- 

 times entirely changed without destroying their character as pebbles, we would refer to 

 another kind of conglomerate, which we have found along the eastern border of Vermont 

 and farther south in Massachusetts. This rock, so far as we know, has never been 

 described in treatises on geology : but we know of four localities along the west side of 

 the Connecticut River, and its character and origin are quite obvious. We reserve the 

 minuter details to another part of our Report, and mention here only the facts that bear 

 specially upon the point under consideration. 



We define this rock as a conglomerate with a cement of syenite or granite, or as a 

 syenite or granite with pebbles in it, sometimes thickly and sometimes sparsely dissemin- 

 ated. We have found it in Whately, Massachusetts, on Ascutney, and in Barnet and 

 Grranby, Vermont. On the southwest point of Little Ascutney we find a conglomerate, 

 or perhaps a breccia, which is made up of fragments of silex and some mica, probably a 

 sandstone, with nearly pure quartz. On one side of this mass it passes, without an 

 intervening seam, into a porphyry, and this into granite, all forming one undivided ledge, 

 so that the conclusion is forced upon us that the granite and porphyry have been formed 

 out of the conglomerate. Most of the rock on Ascutney takes hornblende into its 

 composition, and thus becomes syenite, and this abounds in black rounded masses, which 

 are for the most part crystalline hornblende with some feldspar, and which are probably 

 pebbles transmuted. At Granby the pebbles, manifestly rounded, are either mica schist 



