148 ANNALS NEW YORE ACADEMY OF SCIENCES 



blue-black by reflected light. In 98, crystals with hexagonal outlines are 

 prominent (Plate XIII, fig. 1). One of these reflects light in such a 

 manner that the three upper faces of a flat rhombohedron can be seen. In 

 61, hexagonal tablets are associated with analcite, heulandite and calcite. 

 It is occasionally found in other slides, but no features of special interest 

 hare been observed. 



During the process of ordinary weathering, hematite is not usually 

 formed, but some hydrated compound of the sesquioxide of iron. This 

 applies to the original basalt and to its secondary minerals. Surface 

 decay results in the familiar rusty brown color from the iron compounds, 

 and the hematite itself partakes of this transformation. It is quite evi- 

 dent, therefore, that the processes which were at work in the formation 

 of the zeolitic and associated minerals were essentially different from 

 weathering. The principal factor involved, so far as the hematite indi- 

 cates, would seem to be difference of temperature. There is probably a 

 fixed point at which transition occurs under ordinary conditions from 

 hematite into hydrated forms of FcjOg, or vice versa, and the presence of 

 hematite indicates that the temperature throughout was in excess of this 

 minimum. 



The presence of a purplish dust of hematite often adds to the beauty of 

 crystal specimens. It occurs both as an outer coating and as inclusions, 

 and the inclusion of hematite in a very finely divided condition may ac- 

 count for the delicate pink or flesh tints which the zeolites often show. 

 There is some question, however, whether in the latter case the phenomena 

 may not frequently be due to a ferric compound in solid solution. It has 

 been found in the thin sections that some of the specimens which show 

 such tints carry inclusions of ferric oxide, but on the other hand many of 

 them exhibit nothing to which the color can be attributed. According to 

 interpretations of mineral analyses which have prevailed until recently, 

 the presence of ferric oxide in a zeolite would at once be attributed to a 

 mechanical impurity. It is now recognized, however, that quite diverse 

 compounds may enter a solid solution. A great deal of work requires to 

 be done to ascertain the limitations of this form of combination and what 

 interpretation is correct for individual cases. 



Pyrite and Chalcopijrite 



The total amount of pyrite and chalcopyrite is small, and their occur- 

 rence is of interest chiefly because of the indication it affords of the nature 

 of the processes of alteration. Their presence is attributed to emanations 

 from the consolidating magma and deposition in crevices. In contact with 



