Crosby. | 376 [January 21, 
logic strings resulting from the deformation of the more yielding — 
adjacent pebbles, which are wrapped about it so as to conform 
closely with its surface. The pebbles appear to have been originally 
all well rounded and they consist largely of quartzite, though 
granite, diorite, petrosilex (?), and chloritic, slaty and epidotic 
rocks and epidote, are represented in their composition. I have 
seen no pebbles which seem to have been derived from a micaceous 
rock. That the pebbles are real pebbles, and not concretions of 
any sort, is shown by the forms of those that retain their original 
outlines, by their varied composition, and by their textures. Every 
pebble is enveloped in a layer of micaceous material, and 
the original paste is to a large extent replaced by this mineral, 
which it seems necessary to regard wholly as a product of the 
alteration which the rock has undergone. 
Briefly stated, the transformation of the conglomerate, where com- 
plete, appears to be as follows: the pebbles, as such, disappear, 
leaving a schistose or foliated structure; the greater part of the paste, 
and probably a portion of the pebbles, are converted into a distinct 
mica or hydromica; and, finally, the quartzose and feldspathic 
materials contained in the original pebbles become more distinctly 
and coarsely crystalline. Thus the puddingstone is changed to 
something which is not easily distinguishable from a coarsely schis- 
tose, micaceous gneiss, such as is found in the neighboring towns of 
Mendon, Blackstone, and Uxbridge. And it is highly interesting to 
observe that these same gneisses possess some of the most striking 
structural peculiarities of the altered conglomerate. For instance, 
I have repeatedly observed in the gneiss that a transverse vertical 
section shows no schistosity, no stratification, only a coarsely granitoid 
aspect ; while on a strike section the schistose structure stands out 
prominently, which shows that in the gneiss, as in the conglomerate, 
the quartz, feldspar,setc., are not in layers, but strings. On account 
of possessing this semi-fibrous structure, the dip of the gneiss is not 
always easy to determine. Simultaneously with the alteration of the 
conglomerate to a gneiss-like rock, the arenaceous and argillaceous 
beds are changed, with less difficulty, to quartzite and mica schist; 
though it is likely that much of the true argillite must always 
remain essentially unaltered, being, for chemical reasons, insusceptible 
of metamorphism. 
As already remarked, conglomerates exhibiting extensive altera- 
tion are known to occur at many points throughout New England; 
