388 E. Hitchcock on the Metamorphism of 
they may become syenite, and are frequently porphyritic by dis- 
_ tinct crystals of feldspar. The cement is syenite, often more 
hornblendic than usual. 
’ When the pebbles are highly crystallized, they become so in- 
corporated 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 these 
are produced from the metamorphoses of pebbles which have 
ome crystalline since they were formed into conglomerate. 
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 meta- 
morphosis of stratified rock. 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 constitutions without essentially altering their external 
form. This might certainly be thoroughly done if those pebbles 
_ Were permeated by water containing in solution powerful chem- 
ical agents. Some of the ingredients might thus be abstracted 
from the pebbles and new ones supplied, if needed to form the 
new compounds. 
In all the cases of pebbles in unstratified rocks described 
above, syenite has formed the matrix. " But at the meeting 
of the American Association at Springfield, Prof. Hubbard of 
Dartmouth College exhibited a specimen of pure white granite 
from Warren in 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 infrequent of all rocks, because hornblende 
schist is very limited. But in the older metamorphic conglom- 
erates, such nodules are the most common of all, and this fact 
furnishes the presumption of their metamorphic origin. 
The facts which we have detailed respecting the occasional 
presence of feldspar pebbles in the Vermont conglomerates and 
especially of the occasional conversion of the cement into gneiss, 
are most probably examples of a change of mineral character 
uring metamorphosis. 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 lamine, any 
other than a chemical origin appears improbable. We cannot 
therefore but regard feldspar in perhaps all cases in the — 
ine rocks, as the result of metamorphism. Silicates probably 
furnished the ingredients, which being abstracted by hot water, 
