1879.] 309 [Crosby. 
quently argillaceous; but a large proportion, however, consist of a 
greenish and somewhat unctuous substance which many observers 
have mistaken for serpentine, but which is easily proved to be alum- 
inous and not magnesian, being some form of the protean mineral 
pinite — a hydrous alkaline silicate of aluminum. 
As a general rule, the pebbles composed of these relatively soft 
and, as it were, permanently plastic materials have suffered an 
extraordinary amount of deformation, appearing usually as thin len- 
ticular plates, or as contorted layers enveloping harder pebbles. At 
several points the pudding stone is almost entirely composed of this 
class of pebbles ; and these, being flattened in parallel planes, give rise 
to an imperfect cleavage, which agrees exactly in dip and strike 
with the cleavage in the slate. In such cases the cleavage exists not 
only in the rock as a whole, but in the individual pebbles as well. 
The best exposure of pinite conglomerate showing we'l-marked 
cleavage is on Central Avenue in Milton, about one-fourth of a mile 
south of the Neponset River. 
Turning now to the question of the distortion of the harder peb- 
bles in the puddingstone, I hope to show that, although, as we should 
naturally expect, the evidence is very much less abundant than in 
the case of the relatively soft pebbles, it is, perhaps, scarcely less 
conclusive. That compressing forces of considerable magnitude and 
efficiency have operated in all parts of the conglomerate is evidenced 
by the phenomenon observable in scores of localities of hard pebbles 
which, not receiving equal support at all points from surrounding 
pebbles, have been fractured transversely. Many pebbles show only 
one fracture, while others are divided by several parallel clefts; and 
as a rule the fragments have experienced slight but unequal move- 
ments in the direction of fracture. 
Prof. W. H. Niles has called attention! to the facts (1) that, in 
certain parts of the puddingstone at least, the fractures, which may 
be considered equatorial with reference to the pebbles, are mainly at 
right angles to the strike of the beds, or in the probable direction of 
the compressing force; (2) that in the polar direction there is often 
a manifest tendency to the extension of the materials of the rock, the 
fragments of the fractured pebbles being drawn asunder, and the 
cement separating from the ends or poles of the pebbles; (8) that 
the cavities thus produced between the linear elements of the pebbles 
1 Proc. Bost. Soc. Nat. Hist., Xv, 1. 
