MUSEUM OF COMPARATIVE ZOOLOGY. 103 



of igneous masses on adjacent sedimentaries. Percival makes frequent 

 reference to it in his Eeport ou 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 fmsion 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 tlie 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 hoj-nstone ; 

 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 Xew 

 Britain. Attention iias 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 



