FENNER, THE WATCHVNG BASALT 115 



amples in hand-specimens. In several thin sections, small remnants of a 

 mineral which is probably scolecite were observed in association with 

 natrolite. The chemical relationship between natrolite and scolecite is 

 somewhat similar to that between chabazite and gmelinite, but the group 

 appears to present an example of a different type of solid solutions, 

 namely, that in which the two components do not form a continuous 

 series of mixed crystals, mutual solubility being limited and the end 

 members being of distinct crystallographic forms.-^ Most of the speci- 

 mens are plainly natrolite. Chlorites, probably of several species, are 

 very abundant, but are more prominent under the microscope than in 

 larger specimens. Serpentine is also found in thin sections, but is not of 

 so frequent occurrence as chlorite. Several minerals occur in small quan- 

 tities and have been identified by optical methods. Albite, garnet and 

 several varieties of green amphibole were thus recognized, and their occur- 

 rence will be described in its appropriate place. 



APPLICATIOX OF THE PHASE RULE 



In reviewing the, list of minerals given, it will be observed that the 

 majority of the species are composed of those elements which form the 

 plagioclase feldspars. Frequently there is an addition of the water mole- 

 cule, but otherwise there is no change except in the relative proportions 

 of the various oxides. Different forms of combination of soda (NajO), 

 lime (CaO), alumina (ALO3), silica (SiO.) and water (HoO) are ac- 

 countable for all of the following minerals : labradorite, albite, quartz, 

 prehnite, pectolite, analcite, chabazite, heulandite, stilbite, natrolite, 

 scolecite and laumontite. In what follows, it is proposed to make a 

 special investigation of the mutual relations of these species, based upon 

 those methods of physico-chemical analysis which are embraced under 

 Gibbs's phase rule. It is desirable, therefore, to make a preliminary 

 inquiry into their composition. 



The plagioclase feldspars: The recent investigations of A. L. Day and 

 E. T. Allen^^ into the nature of the plagioclases, carried out in the Geo- 

 physical Laboratory at Washington, have shown almost beyond question 

 that the albite and anorthite molecules form a continuous series of solid 

 solutions of the type in which mutual solubility is unlimited and in which 

 the fusing points of intermediate members lie between those of the 

 isomorphous end-compounds. All the members from albite, XaoOAUOg- 



=* For analogues among artificial salts, see Alexaxdeh Findlay : "The Phase Rule and 

 its Applications," New York, p. 180, 1006. 



=»Am. Jour. Sci., 4th ser., vol. 20, pp. 52-70, 1000. 



