806 HISTORICAL GEOLOGY. 



over the region west of the ridge. It follows therefore that the eruptive 

 work began before the close of the period of the sandstone formation. 



But it appears to be also true that it characterized the closing part of the 

 period. The facts from West Eock, and others of similar import from East 

 Haven, where the trap rests on upturned sandstone, are evidence that so far 

 as these regions are concerned, the upturning preceded the eruptions. This 

 conclusion involves the Saltonstall region ; and if this, so also the Totoket 

 Ridge and others to the north, since all are closely alike, and in close con- 

 junction. Moreover, a laccolithic origin may be inferred not only for East 

 and West Rock, but for all such cases. 



With regard to the Mount Tom Eidge, direct evidence of age of eruption 

 is wanting ; for no east-and-west sections have been reported. But a lacco- 

 lithic origin and the abrasion of the underlying sandstone are indicated by 

 the occurrence of breccia beneath the trap, and especially by the limestone 

 chips in the loAver part of the mass of the trap, a.nd also over its upper 

 surface, as described by Emerson. A bed of limestone was evidently divided 

 by the advancing tongue of melted trap, part being left below, and the rest 

 above. As Emerson observes, the facts prove that the heavy trap flowed 

 over the sandstone, abrading and tearing it. But they prove also that the 

 flow was not surficial, but laccolithic ; for in the case of an advancing surficial 

 stream the lava, being retarded by friction at bottom, has a downward flow 

 at the front, and hence could not bear to its upper surface material met with 

 along its track. 



A laccolithic origin for the Mount Tom Bidge explains also the existence 

 of the attendant dike parallel with its southern, western, or eastern side, and 

 for similar cases elsewhere. Eor whenever, in the forced flow of lava from 

 the supply fissure to make the laccolith, the force could not so easily con- 

 tribute to the laccolithic mass (owing to the weight it had acquired by 

 accumulation and that of the overlying sandstone, and to resistance froin 

 other sources) as make a fracture either side for a new place of escape, the 

 latter event would take place. A dike of five inches, which is visible under 

 the trap mass in the south front of West Rock, and which is both amygda- 

 loidal and chrysolitic, is probably an example of this mode of origin. 



This evidence of a laccolithic origin brings the north-and-south trap belts 

 into the same category, as to method and time of origin, with West Rock 

 and East Rock. After or during the upturning of the sandstone appears, 

 therefore, to be the time of origin of the larger part of the eruptions. 



The hypothesis has been brought forward by W. M. Davis {U. 8. G. S. Bep., vol. 

 vii., and elsewhere), that the larger part of the trap was erupted in the early part of the 

 Triassic period long before the upturning ; that in the case of the Connecticut valley 

 area, the trap was poui'ed out surficially from fissures along the eastern margin of the 

 area, and thence flowed westward across it over the underlying sandstone ; that after more 

 sandstone had been deposited a second and larger surficial flow took place ; then after 

 more deposition of sand-beds a third smaller flow ; and that this interstratified sandstone 

 and trap were covered by other horizontal deposits of sandstone of great thickness ; that, 



