FEN^'ER, THE WATCHUNG BASALT 10:j 



basalt, sheathed with glassy crusts, appear in those areas which, at the 

 time of the lava flow, were covered by shallow lakes, and that these areas 

 are coincident with those in which the formation of zeolitic minerals has 

 occurred. 



Evidence along various lines leads to 'the belief that the processes of 

 alteration were directed by the features impressed upon the solidified 

 basalt by the presence of the bodies of water. Among the spheroidal 

 masses, a considerable amount of interstitial space had been left. More- 

 over, the crusts were much shattered, and frequently the interiors of the 

 masses were penetrated by a multitude of cracks produced by shrinkage 

 in cooling. Where the openings were of sufficient size to permit the free 

 passage of superheated aqueous vapors from the water-impregnated sedi- 

 ments beneath, these gases appear to have rushed upward with great force 

 and velocity, carrying with them quantities of finely comminuted dust 

 from the lake beds and depositing it in the various interstices, in the 

 form of a reddish-brown powder. Much of this is probably of a clayey 

 nature, but it is so fine that it is not resolvable under the microscope, 

 though some of it shows also grains of quartz and feldspar such as are 

 found in the coarser sediments. Mingled with this material, there are 

 many fragments of basaltic glass, whose nature is easily recognized. In 

 some cases, these deposits appear to have filled open spaces of considerable 

 size and in others to have penetrated minute cracks. At a quarry for- 

 merly worked near Great Notch, the igneous rock carries between the 

 bowlder-forms irregular blocks of this character, ranging in size up to a 

 foot or more in diameter, at first sight counterfeiting inclusions of fine- 

 grained sandstone. Examination under the microscope, however, shows 

 the presence in this pseudo-sandstone of a notable admixture of glass, and 

 the manner in which the material ramifies into cracks of the basalt sug- 

 gests its origin. Hand-specimens may sometimes be found, in which the 

 peculiar relation appears of small dikelike bands of reddish sandstone 

 cutting the basalt. Tliis reversal of the usual relations is interesting 

 from its unusual character. 



The formation of such deposits appears in some instances to have sealed 

 the larger spaces left open after the consolidation of the igneous masses, 

 but nevertheless the brecciated material around the bowlders and the 

 multitude of shrinkage-cracks often present would offer spaces of capil- 

 lary size or larger and would undoubtedly render the piled-up masses of 

 bowlder-like structure above the lake beds distinctly more porous and 

 susceptible to the circulation of water than the dense, massive basalt 

 beyond the borders of the lakes. Field observations lead to the conclusion 

 that such is clearly the case. 



