160 ANNALS NEW YORE ACADE2IY OF SCIENCES 



Further evidence of the more intense activity of the metamorphic 

 processes in the earlier stages is supplied by the fact that the holocrystal- 

 line phase of the basalt was undoubtedly attacked and recrystallized to a 

 certain degree and that the phenocrysts of diopside and plagioclase left 

 little or no trace. In the later stages, recrystallization appears to have 

 acted only upon the glass or upon aphanitic basalts whose crystallization 

 was of the hairlike microlitic type, and remnants of phenocrysts persist 

 for a long time. 



Secokd Period of Altekation 



Progress of the Changes 



At this point, a marked change occurs in the nature of the minerals 

 deposited. The differences between the two periods have been ascribed, 

 both on theoretical grounds and on the evidence of the microscopic sec- 

 tions, to falling temperature and to the disturbing influence of boric 

 acid in the solutions during the first period and its elimination during 

 the second period. 



In the primary consolidation of the magma, plagioclase was present m 

 abundance. During the first period of alteration, the anorthite molecule 

 was immediately broken up to form new compounds to which it bore no 

 resemblance in chemical structure. The albite molecule persisted for a 

 short time, but soon followed a similar course. The molecular groupings 

 characteristic of the plagioclases were destroyed. The components en- 

 tered into such compounds as garnet, arfvedsonite, prehnite, pectolite 

 and datolite. Other compounds may have been formed in the solutions, 

 but they did not reach the point of crystallization, unless represented in 

 minerals which subsequently disappeared completely. 



During the second period, there was no further source of boric acid 

 which could be drawn upon, and that which had been deposited as dato- 

 lite was gradually taken up by the solutions and removed. The effects 

 of the presence of boric acid disappeared. Ferromagnesian constituents 

 also became greatly diminished. Feldspathoid compounds reappeared, 

 and it seems as if the conditions approached those of a closed system in 

 which feldspathoid combinations alone were present and in which the 

 only independently variable factor of importance was temperature, which 

 gradually diminished. As a result, the feldspathoid group of the zeolites 

 formed the principal constituents deposited during the second period. 

 Coincident with the change in the chemical nature of the minerals, a 

 change in physical characteristics appears. The minerals of the first 

 period were predominantly rather hard and of fairly high relief and 



