HEAT. 303 



Among the trap-ridges of the Connecticut Valley, East Rock (page 298) 

 is of laccolithic origin. The supply-fissure for East Rock dips eastward at 

 about 45°. The liquid rock on passing up the fissure between the sandstone 

 walls, whose beds also dip eastward, but at an angle of 20° to 25°, forced a 

 passage westward between the beds of the sandstone, and made a mass of 

 trap 200 to 250 feet or more in thickness, and about 300 yards in breadth. 

 It is known to be laccolithic by the fact that the sheet of trap keeps its 

 thickness quite to its extreme western limit, instead of thinning by gravity, 

 like a surficial flow, and that it has also a rising slope throughout. The 

 section. Fig. 274, represents the intrusion of trap, from an oblique fissure, 

 between layers of sandstone, in laccolithic style ; and the removal of the 

 overlying sandstone would give it a general resemblance to a section of East 

 Eock. But in East Rock, and also in West Rock, of the same region (see 

 map, page 299), the trap of the outflow rests on the edge of upturned layers 

 of sandstone, and it has less dip than the sandstone. The condition in East 

 Rock is shown in Fig. 275, and that in a second summit of the East Rock 

 Ridge, in Fig. 276. Fig. 277 represents part of a long exposure of the up- 

 turned sandstone in the south front of West Rock — a transverse or east- 

 and-west section of the Rock. Above the sandstone, only the basal portion 

 of the columnar trap is shown in the figure, and below it, a talus of fallen 

 stones and earth. The forced laccolithic flow of the liquid rock under its 

 heavy cover of sandstone must have caused the abrasion of the fragile 

 underlying beds. 



In some cases in the Connecticut Valley, portions of the sandstone and trap, at the 

 contact, occur rolled into rounded forms, and make part of an intervening layer between 

 the trap and sandstone. The resistance produced by the weight of sandstone above fre- 

 quently caused the opening of parallel fissures, for the escape of the lavas ; and the rock 

 of these outflows is often amygdaloidal , when the rest is not, owing to the accumulation 

 of subterranean waters produced in consequence of the damming by the descending dike. 



Unsolved Questions about Igneous Phenomena. 



1. Origin of the ascensive force. — The ascensive force in the volcano 

 has been attributed to (1) the expansive action of moisture from the deep- 

 seated source of the lavas ; and (2) the gravitational pressure of the con- 

 tracting crust of the globe, forcing up the lavas ; and some of the very deep 

 depressions in the ocean's bottom near volcanic islands are thought to favor 

 the latter theory. In view of the fact that the central part of a lava column 

 should be the hotter, it is queried whether there is not, owing to the 

 ascending vapors, a more rapid rising along the center, and a consequent 

 descending along the sides of the conduit. 



The facts afforded by Kilauea indicate that the upward movement in a lava column, 

 as a consequence of the ascensive force, is very slow — 360 feet in 6 years being the 

 maxinmm observed (page 280). It appears also to follow from the facts, as stated on 

 page 276, that the force in the conduit varies with the amount of moisture received from 

 descending subterranean waters. Daubree, whose experiments on the perforating power 



