FENNER, THE WATCHU^iG BASALT 165 



In slide 94, the chief constituents are analcite, chabazite and heulan- 

 dite. The last two appear perfectly fresh, while the analcite is turbid 

 and shows evident corrosion in contact with the other two. 



In 92, the first period is represented by remnants of albite, green 

 amphibole and datolite, which have probably replaced aphanitic basalt. 

 The second period is represented by analcite and stilbite, with a little 

 heulandite, and the third period by calcite. The analcite, which has 

 become very turbid and is quite decidedly anisotropic, has been corroded 

 and replaced by stilbite. Calcite appears to replace everything. 



In 61, the analcite is found in areas of considerable size, in some places 

 perfectly clear and isotropic and in others showing turbidity and incip- 

 ient alteration. At one place, calcite is encroaching upon the analcite. 

 In a hand specimen, large crystals of analcite are covered with a solid 

 crust of stilbite, composed of tabular crystals disposed normally to the 

 crystal faces of the analcite. There is no doubt of the later deposition 

 of the stilbite. 



These relations of analcite to chabazite, heulandite and stilbite are in 

 mutual accord and appear to make the period of analcite earlier than 

 that of either of the others. This may, however, have been true in only a 

 limited sense. Analcite is a soda compound, whereas chabazite, heulan- 

 dite and stilbite are essentially lime compounds, and the period of anal- 

 cite may have overlapped those of several lime zeolites. If such were the 

 case, a slight difference in the relative concentration of soda and lime in 

 the solution might suffice to cause the deposition of analcite at one point, 

 while it was being removed at another. One of the hand specimens shows 

 a reversal of the order of analcite and heulandite, probably thus explain- 

 able. Large cockscomb crystals of heulandite have small crystals of 

 analcite resting upon their surfaces. There seems to be no doubt that 

 this analcite is of later deposition. 



The relations of analcite and natrolite are of especial interest because 

 of the inferences derived from the application of the phase rule. It 

 should be found that where the two exist together, one belonged to a 

 period of higher temperature and remained stable until a definite tran- 

 sition point was reached, when it began to pass over into the other. It 

 appears that this is true and that the direction of the change is from 

 analcite to natrolite. The relations are brought out in slide 82 (fig. 24). 

 In the specimen from which this was prepared, small nests in a mass of 

 porcelainlike natrolite contained crystals of analcite and laumontite, 

 easily recognizable macroscopically. With these, natrolite crystals of 

 much larger size than the general average of the mass were mingled. In 

 the thin sections, the analcite appears spongy and decomposed and has 



