SYENITIC CONGLOMERATE. 41 



or white almost hyaline quartz, just such as form the pebbles in the conglomerates at 

 Wallingford and Plymouth, and the base is a fine grained syenite, passing sometimes 

 almost into mica schist. A pebble of hornblende schist is also sometimes seen. 



In bowlders of this conglomerate found in Northampton, Mass., and probably derived 

 from Whately, the most abundant pebbles are those of the brown sandstone, considerably 

 metamorphosed and flattened. Those of hornblende schist are common. Sometimes 

 they are merely crystalline hornblende, not foliated generally, however, but mixed with 

 some feldspar, and they may become syenite, and are frequently porphyritic by distinct 

 crystals of feldspar. The cement is syenite, often more hornblendic than usual. 



When the pebbles are highly crystallized they become so incorporated with the matrix 

 that it is difficult to separate them with a smooth surface, and if we are not mistaken 

 they pass insensibly into those rounded nodules, chiefly hornblendic, so common in 

 syenite, especially that of Ascutney. We think those are produced from the metamor- 

 phosis of pebbles, which have become crystalline since they were formed into conglom- 

 erate. We find them as we think in all stages of the metamorphosis. 



These facts certainly give great plausibility to the view which supposes granite 

 and syenite to be often the result of the metamorphosis of stratified rocks, as we 

 shall more fully explain in another place. But they afford a presumption, also, in favor 

 of the position that pebbles, which have been plastic for ages in the rocks, may have 

 greatly changed their mineral constitution without essentially altering their external form. 

 This might certainly be thoroughly done if those pebbles were permeated by water con- 

 taining in solution powerful chemical agents. Some of the ingredients might thus be 

 abstracted from the pebbles and new ones supplied, if needed to form the new com- 

 pounds. 



In all the cases of pebbles in unstratified rocks described above, syenite has formed the 

 matrix. But at the meeting of the American Scientific Association at Springfield, Prof. 

 Hubbard, of Dartmouth College, exhibited a specimen of pure white granite from War- 

 ren, New Hampshire, in which there lay imbedded a rounded bowlder of hornblende rock, 

 more than a foot in diameter, and easily separable from the granite. We had no doubt but 

 that it was mechanically rounded, nor much doubt but that its mineral character had been 

 changed since it was enveloped in granite. Hornblende bowlders in the drift are among 

 the most unfrequent of all rocks, because hornblende schist is very limited. But in the 

 older metamorphic conglomerates such nodules are the most common of all, and this fact 

 furnishes a presumption of their metamorphic origin. 



The facts which we have detailed respecting the occasional presence of feldspar pebbles 

 in the Vermont conglomerates, and especially the conversion of the cement occasionally 

 into gneiss, are most probably examples of a change of mineral character during metamor- 

 phosis. It seems hardly possible to account for a cement of crystalline mica or talc in any 

 other way. But when we find feldspar interpolated between the folia, any other than a 

 chemical origin is superlatively absurd. We cannot, therefore, but regard feldspar, in 

 perhaps all cases in the crystalline rocks, as the result of metamorphism. Silicates 

 probably furnished the ingredients, which being abstracted by hot water, left the excess ot 

 silica in the form of quartz, and formed the feldspar and mica to fill up the interstices. 

 The feldspar which has converted the cement into gneiss could have had no other origin, 



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