MINEEALOGY; WITH A CLASSIFICATION OF SILICATES. 59 



Connecticut, includes anorthite, of which the larger feldspathic grains are composed, and 

 showed by calculation that the rock as a whole might contain in 100.00 parts, anorthite 

 15.52, albite 22.16, orthoclase 2.82, the remainder being pyroxene with a little menac- 

 canite and magnetite, apatite and some combined water being present. 



A farther attempt to determine the composition of the feldspathic element in a simi- 

 lar diabase was then made with that from the Palisades of the Hudson at Jersey City. 

 The feldspar from this, separated from the other ingredients by the aid of a solution of 

 potassio-mercuric iodid of specific gravity 2.90, was, by a similar solution of 2.69, clearly 

 divided into two portions, a lighter or a heavier, which gave by analysis respectivel}^ 

 the composition very nearly of andésite and of labradorite, — the atomic ratios for the first 

 portion being 1:3:8 and for the second 1 : 3 : 6J, without including 0.59 of water in the 

 first, and 1.06 in the second. Hawes reasons with regard to these liicts, that slight varia- 

 tions in the conditions of cooling in a fused mass might determine either the separation 

 of the feldspathic element as the two species, anorthite and albite ; or the formation of 

 one or more intermediate species. He adds, with rare insight, " an exquisite balance of 

 composition and circumstance woirld be necessary to crystallize such a rock with a single 

 feldspar," and conceives that we have evidence that massive rocks are rarely simple as 

 regards their feldspathic element.' Meanwhile it will remain to be decided for each indi- 

 vidual case, whether a feldspathic material intermediate in composition between albite 

 and anorthite is an integer, or an admixture of two integers, which may themselves be 

 either the terms of the series or integral intermediate species. 



Mention should here be made of petalite, a species in many resi^ects closely 

 related to the feldspars, but presenting the ratios, 1 : 4 : 20. As regards the proportion 

 between protoxyds and alumina, it is important as the one spathoid which corresponds 

 with the rare and less protobasic zeolitoids ; while if krablite be rejected, petalite is the 

 most siliceous species known. Its atomic volume is identical with that of anorthite, 

 albite and iolite. 



§ T5. The scapolites apparently constitute a single genus of silicates which approach- 

 ing in composition, hardness and density the feldspars, were, from an early time, com- 

 pared with them, so that when, in 1854, the writer attempted to generalize the notion of 

 Yon Waltershausen as to crystalline intermixtures in the intermediate feldspars, he 

 extended a similar view to the scapolites, as already shown (§ 31). The ratio between the 

 protoxyds and alumina in the scapolites has, until recently, generally been regarded as 

 1 : 2, and the writer, in 1863, in farther discussing the relations of the scapolites, described 

 them as a group of which the extreme terms were meionite, with the ratios 1:2:4, and 

 dipyre with 1 : 2 : 6 ; including intermediate species which might be regarded as crystal- 

 line admixtures of the two isomorphous silicates. 



Very recently, however, Tschermak has reviewed the scapolites,- and has reached 

 the conclusion that the atomic ratio of the protoxyds to alumina therein is not 1 : 2 as 

 hitherto supposed, but 4 : 9 or 1 : 2|. In other words, if we would compare them with 

 the feldspars by multiplying their atomic formulas so as to get in each the same amount 

 of silica, while anorthite becomes (ca3al,jsii2)o24, meionite is not (caialssii2)oo„ but 



' Proc. U. S. National Museum for 1881, pp. 129-134. 

 ^ Monataliefte fur Chemie, Dec, 1883. 



