FENNER, THE WATCHUNG BASALT 109 



forces by which the mechanism is urged forward. A little further insight 

 into this is permitted by the fact, which has been ascertained to be true 

 in many instances and is believed from thermodynamic considerations to 

 be a general law, that in any association of compounds, the stable forms 

 are those for which, under the given conditions of temperature, the vapor 

 pressures are a minimum. The vapor pressures of solids are generally so 

 minute as to appear almost infinitesimal, but are not therefore negligible. 

 Even without the intervention of a medium of solution, molecular re- 

 arrangements take place. This is well established for metallic alloys and 

 sometimes occurs with natural minerals. In general, however, a solvent 

 is requisite, and in its presence the compounds having higher vapor pres- 

 sures pass more readily into solution and those with least vapor pressures 

 crystallize out. 



The principle is of immediate applicability in explaining the facility 

 with which the glassy crusts of the basalt were attacked by the circulating 

 waters. A glass, as is well known, is, from the standpoint of physical 

 chemistry, a greatly under-cooled liquid, in which crystallization has been 

 checked by the viscosity. Its metastable condition is expressed in the 

 excess of its vapor pressure over that which the component minerals 

 would possess in their crystalline phase. It should therefore pass into 

 solution more readily than the normally crystalline basalt, and such 

 seems undoubtedly to have been the case. When the structure of' the 

 deposits is examined in the field, it is found that the secondary minerals 

 form nests and pockets in the angular spaces between adjacent bowlder- 

 forms and wrap around them in bands in the situation in which the 

 glassy crusts were originally developed. This is in part due, no doubt, to 

 the fact that channels of easiest circulation followed such features, but 

 the whole effect cannot be ascribed to the latter cause, for cracks pro- 

 duced by shrinkage or by deformation and passing through the interior 

 of the bowlders do not exhibit an equal development of alteration prod- 

 ucts. Confirmatory evidence of selective alteration of glass is found in 

 the slides. In a number of instances, crystalline crusts of later minerals 

 show structures which seem best explainable as survivals of original forms 

 in glass. Eecrystallization seems to have been checked, however, when 

 the normal basalt was reached, and at times its effects almost disappear 

 within the area of the thin section. 



Slide 53 affords an instructive illustration of several of these points, as 

 is shown in Plate XI, fig. 5. The microscope shows crusts of recrystal- 

 lized minerals composed of prehnite, green amphibole, specularite, sec- 

 ondary albite and garnet. Although recr\'stallization has produced large 

 crystals, an indication of a former structure survives in certain curved 



