the Earth' 's Contraction from Coolinf/. 281 



facts illustrating this kind of metamorphism yielding chloritie 

 and zeolitic minerals. 



There is hence nothing in the relations of the different kinds 

 of doleritein the Connecticut valley or any part of the Triassico- 

 Jurassic regions of the Atlantic slope, cither as to geographical 

 position or chemical composition, that favours the idea of differ- 

 ence of original subterranean source. 



Now such a similarity of product occurring at intervals over 

 a region 1200 miles long, and in several parallel lines, shows 

 some unity of origin. It evinces that the source must have 

 been either the undercrust fire-sea, like the under-Appalachian 

 as we have termed it, derived from the old viscous layer, or else 

 plastic masses within the true crust, which crust, as it was made 

 from the viscous layer by cooling, would have like uniformity 

 of mineral constitution. 



This example of igneous eruption is grand in scale, since the 

 outflows are from tappings along a line more than a thousand 

 miles long ; and it may therefore serve well to test the general 

 application of theories of igneous eruptions. It indicates that 

 the original fire-sea was one of great extent. 



Professor T. Sterry Hunt, in his paper " On the probable 

 Seat of Volcanic Action"*, and others of earlier date, while 

 adopting the view of a nucleus made solid by pressure and of an 

 exterior crust, with a viscid layer between, has proposed to sub- 

 stitute for the viscous layer recognized as the source of volcanoes 

 by Hopkins, one made by the " aqueo-igneous fusion" of part 

 of the lower sedimentary beds of the supercrustf. The alleged 

 causes of such fusion are : — (1) pressure due to the accumulation 

 of superficial sedimentary beds ; (2) increase of heat, by a rise of 

 the isogeothermals — another effect of the same cause; and (3) 

 the presence of moisture, a common feature of sediments — the 

 moisture becoming, under the heating, superheated vapour, 

 and so aiding by its great dissolving power in the result. In a 

 later article J he says, " I have taken pains [informer papers] 

 to explain that the deeply buried layers of sediment, together 

 with the superficial and water-impregnated portions of the solid 

 nucleus, constitute a softened or plastic zone from which all 



me in the conclusion. In the paper referred to I attribute the change to 

 infiltrating waters. The opinion has many advocates. 



* Silliman's Journal, S. 2. vol. 1. p. 21 (1870). 



t I use the term supercrust as it is explained on page 216— 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. 



% Geological Magazine, February 1868, 



