108 J. D. Dana—Results of the Earth's Contraction. 
There is hence nothing in the relations of the different kinds 
of dolerite in the Connecticut valley or any part of the Trias- 
sico-J urassic regions of the Atlantic slope, either as to geograph- 
ical position or chemical composition, that favors the idea of 
difference of original subterranean source. 
ow such a similarity of product occurring at intervals over 
a region 1,200 miles long, and in several parallel lines, shows 
some unity of origin. It evinces that the source must have 
been either the under-crust fire-sea, like the under-A ppalachian 
as we have termed it, derived from the old viscous layer ; oF 
else plastic masses within the true crust, which crust, as it was 
made from the viscous layer by cooling, would have had like 
uniformity of mineral constitution. 
This example of igneous eruption is grand in scale, - 
D 
Professor T. Sterry Hunt, in his paper on “The probable 
seat of volcanic action,”+ and others of earlier date, while adopt- 
» Pogg. Ann., Ixxxiii, 197-272, 1851. The secondary origin of the zeolites and 
chlorite found to penetrate some dolerite and other kinds of igneous rock, as well 
as filling t ithin them, I sustained in an article in vol. xlix of the first 
series of this Journal (1845); and the various facts I have since observed have 
nded to confirm me in the conclusion. In the paper referred to, I attribute the 
ae ne 
the term supercrust as it is explained on page 12, that is, for the part 
of the crust made by the action of exterior agencies over the outer surface of the 
true crust, the true crust being the part that was formed directly from cooling. 
