186 ANNALS NEW YORK ACADEMY OF SCIENCES 



the surface through the interstices of the basalt in the situations de- 

 scribed. 



Eecrystallization was probably begun as soon as the lava had cooled 

 sufficiently to permit the waters to hold a liquid form. A considerable 

 degree of superheating was permitted because of the head to which they 

 were subject. By degrees, however, the whole mass of lava became cooler, 

 so that in the final stages of alteration, the temperature may have been 

 but little above the average climatic temperature of the region. 



The majority of the secondary minerals are found to consist of the 

 elements present in the original plagioclase feldspar, with or without the 

 addition of water. The possibilities of equilibrium among the great 

 number of compounds present was considered in the light of Gibbs's phase 

 rule, and some inquiry was made into the applicability of this rule to the 

 conditions of the problem. It was deemed most probable that, although 

 various modifying conditions entered whose effect could not be wholly 

 taken into account, the general conclusions to which the phase rule 

 pointed should hold and that the possibility of all the compounds present 

 coexisting in equilibrium could not be admitted. 



Petrographic examination of thin sections confirmed these conclusions, 

 and the sequence of minerals and the form taken by processes of replace- 

 ment have been described at length. It was found that in addition to the 

 zeolitic minerals which form such a prominent feature of the occurrence, 

 and to such minerals as prehnite, datolite, quartz etc., which are present 

 in notable amount, traces of other minerals have survived. Such are 

 albite, garnet, arfvedsonite etc., which represent the effects of intense 

 metamorphism in the early stages of recrystallization. There is reason 

 to believe that these were formed in considerable quantities, but were 

 almost destroyed during succeeding stages. 



It was found possible to trace very perfectly the alteration of the glass, 

 or less often the normal basalt, into various associations of secondary 

 minerals ; and several features present in the original rock were found to 

 have left traces through all stages of recrystallization. In some instances, 

 the theory of solid solutions was of assistance in affording a basis of 

 explanation of the changes which minerals had undergone. This was 

 especially true in the case of the development of albite from original 

 labradorite and in the transformations of the amphiboles. 



As a final effect of alteration, quantities of calcite were formed. The 

 greater part of this is believed to have been produced at temperatures but 

 little above the ordinary. The presence of the carbonic acid is attributed 

 to a derivation from the atmosphere at the beginning of the cycle of cir- 

 culation. 



