190 G. W. Hawes—Trap Rocks of the Connecticut Valley. 
ferent composition ; for a very slight change in the composition 
of the whole mass would account for the formation of the small 
amount of anorthite which the rock contains, and the differ- 
ence in its fusibility, that of labradorite being 3, and that of 
anorthite 5, would favor the separation of the anorthite. 
2. Diasasz, on Cutortric Do.erire. 
In the examination of the various dikes we find the rocks in 
any amygdaloidal cavities, that the sample was selected. 
LaKE SALTONSTALL. Sp. gr. = 2°86. 
L It. Mean. 
Sieh ie ST 49°29 49 28 
Alumina soos 15°87 15°97 15°92 
Ferrous oxide... 10°17 10°23 10°20 
Ferric oxide __.. 1°93 1°88 1°91 
Manganous oxide °35 40 ‘87 
AR ee was, 7-46 7°42 7°44 
Magnesia.-..... 5°90 6°07 5°99 
ik Se ome oe ee 3°36 3°40 
A ag: = Tage i Se "74 69 72 
Water’: i222. 3°92 3°88 3°90 
Carbonic acid _. 1°12 1°17 114 
100718 100°36 100°27 
A specimen from the southern dike of a high ridge called the 
Durham Mountains exhibits still greater alteration, for the 
amount of water shows that the larger part of the pyroxene has 
been changed to chlorite. 
It will be seen that the alteration of these rocks has not beet 
attended by further oxidation of the iron, and therefore it could 
not have been accomplished by any surface action, since the 
oxidation of protoxide of iron is one of the chief causes of sur 
face alteration; while in this case one mineral containing pr 
toxide has been changed into another protoxide mineral. It 
would, therefore, seem certain that the alteration took place a 
