138 ANNALS NEW YORK ACADEMY OF SCIENCES 



strongly in a band lying between the quartz and the basalt, and it might 

 appear at first that it was earlier than the quartz, but its general char- 

 acter and the manner in which it branches minutely into the quartz are 

 indicative of replacement. In addition, certain features shown in fig. 13 

 can be explained only in this way. Inclusions, plainly original in the 

 quartz, survive within the replacing calcite. 



This form of replacement, by which later calcite has worked in between 

 crystallized quartz and basalt, seems to have been very common. - Another 

 good example is found in slide 41, and in repeated instances hand speci- 

 mens are found to show a narrow band of calcite between the two, which, 

 without microscopic examination, would naturally be inferred to be 

 earlier than the quartz. A similar development of calcite as regards 

 other minerals is referred to elsewhere. 



It is not easy to understand why a deposition of calcite should effect 

 the simultaneous removal of quartz or why the solution of quartz should 

 cause the deposition of calcite, since the two minerals have no ion in 

 common. It is known, however, that the solubility of a salt in water is 

 often diminished by the presence of another salt, even when there is no 

 common ion. 



The observed relations indicate that quartz is earlier than datolite, 

 prehnite, pectolite, chabazite, heulandite, stilbite, apophyllite and calcite. 

 It is referred therefore to the first period of alteration. 



Garnet 



In several of the thin sections, numerous small grains of some mineral 

 of high relief and hexagonal or rounded outline are found, regarding 

 whose identification there is some slight doubt, but which may be referred 

 to garnet with most probability. They occur normally in association with 

 that group which is believed to have been the result of the most intense 

 metamorphic action. In hand specimens, individual grains cannot be 

 distinguished, but the presence of small clumps of some substance of a 

 peculiar, light, ashy-green color in certain associations have been found 

 to be due to a multitude of grains of this character. 



One of the best examples is slide 98. The relations are sketched in 

 Plate XIII, fig. 1. The garnet is here associated with albite, actinolite, 

 specularite and prehnite, forming crusts on dense basalt. The grains in 

 juxtaposition form considerable aggregates, or they are scattered in 

 swarms through the prehnite. The average diameter of individual grains 

 is 0.02 mm., and the maximum is not much greater. Each individual 

 consists of a clear, colorless exterior and a darker nucleus of a brownish 



