MUSEUM OF COMPARATIVE ZOOLOGY. 103 
of igneous masses on adjacent sedimentaries. Percival makes frequent 
reference to it in his Report on the Geology of Connecticut. Yet, of all 
the above mentioned signs of intrusive sheets, it is perhaps the most 
difficult one to recognize. Simple induration is easily enough deter- 
mined with a hammer; but it is another thing to decide whether it 
results from well advanced cementation by minute deposits of calcite 
or quartz brought by infiltrating waters, or from baking by heat. The 
sandstone overlying Saltonstall or Pond Mountain at its northern end 
is excessively hard; but its hardness is due to secondary deposits of 
calcite, and not in the least to fwsion or baking. Moreover, it fre- 
quently happens that the beds overlying undoubted intrusions or ad- 
joining dikes are not hardened: this is commonly the case with 
sandstones, as, for example, on the back of Gaylord’s Mountain. Shales 
are more affected by a dehydration of their clayey constituents, new min- 
erals being formed when the temperature is higher and water abundant. 
Sections cut from ordinary biscuit-ware show under the microscope no 
essential difference from the hydrous kaolinite from which the ware was 
made, excepting a greater compactness. The argillites of Somerville, 
Mass., manifest little local alteration near their abundant dikes; as if 
the general metamorphic process which changed the original clay-beds 
into argillite had been so complete that the comparatively slight local 
influence of the dikes was not sufficient to carry the change any further. 
The argillites of Quincy, Mass., contain small garnets close to the large 
intrusions of the Blue Hills. The shales overlying the Palisade Range 
have been changed in color and texture so’ as to resemble hornstone ; 
biotite, hornblende, and epidote have been locally developed. 
The induration of the sedimentary rocks immediately overlying the 
trap sheets has not been neglected in the study of the ridges ; but while 
simple induration is associated in some cases with unquestionable signs 
of intrusion, it is found in other cases with equally decisive indications 
of extrusion, and we have therefore been driven to the belief that mere 
induration is by no means of constant occurrence or definite association, 
and that it must be regarded as of little determinative value, at least 
for the Connecticut eruptives. 
Our search for evidence of the origin of the trap sheets has been car- 
ried from the coast of the Sound, by New Haven and Branford, along 
the greater part of the various trap ridges, to Cook’s Gap, west of New 
Britain. Attention has been given chiefly to the back of the sheets, 
for the upper contacts are much more significant than the lower; but, 
although the upper contact lines must. altogether amount to one or two 
