Fairbanks.] 



A nalcite Diabase. 



285 



which are idiomorphic. He says : "Nepheline could, of course, not 

 be the mineral from which the analcite might be derived, the form 

 of the crystals prohibiting that supposition." It would appear to 

 the writer that this could be no objection, as nepheline frequently 

 shows hexagonal sections. No evidence is at hand to support the 

 view of its subsequent introduction into the rock. Its apparently 

 uniform distribution through the whole of the extensive eruptive 

 mass which has not been fissured or sheared makes this view seem 

 utterly untenable. Although no direct evidence appears in the 

 rocks under discussion in favor of the supposition of the derivation 

 of the analcite from an unknown soda-rich mineral, it seems prob- 

 able that such is its origin. The test for sulphur gave a negative 

 result, while that for chlorine resulted in only a trace. The min- 

 erals of the sodalite group are thus excluded. Nepheline is the only 

 remaining soda mineral from which it might be derived, and it will 

 be shown how closely the rock resembles some from other locali- 

 ties known to contain nepheline or eleolite. A slide from one of 

 the smaller dikes exhibited a number of areas of an isotropic sub- 

 stance partly inclosed in the feldspars and idiomorphic with ref- 

 erence to them. Several of these have a rude hexagonal outline, 

 and one in particular is very regularly hexagonal. (Plate 1 5, Fig. 5.) 

 They are not apatite, as is shown by the partial decomposition and 

 absence of an interference figure, and are, without doubt, referable 

 to analcite. Scattered through them are small crystals of a sub- 

 stance which has the character of calcite. Analcite also occurs in 

 the same rock, in wedge-shaped areas between the feldspars. In 

 the one case the analcite has certainly replaced a soda mineral, 

 perhaps nepheline, and in the other its origin is doubtful. It is 

 possible a part of the nepheline in the dike crystallized before the 

 feldspar, and the remaining portion, if such existed, as the last con- 

 stituent. The feldspar, judging from the extinction angle, all 

 belongs in the basic portion of the series. Even on the supposition 

 that it were albite of normal composition, and form fifty per cent, 

 of the rock, the percentage of soda which should be present in the 

 rock would fall below that given in the analysis. Confirmatory, 

 then, of the view of the original derivation of the analcite from 

 nepheline is the relative basicity of the feldspar and the high per- 



