164 ANNALS NEW YORE ACADEMY OF SCIENCES 



The supposition that the bands represent original cracks is confirmed 

 in another portion of this slide, where a very fresh-looking diopside 

 crystal has been broken across. The two parts are now separated about 

 0.2 mm. and have been faulted slightly out of line, but still extinguish 

 in unison. The faulting band is filled with chlorite and a small amount 

 of ferruginous sediment from external sources. These cracks undoubtedly 

 formed the passages along which the heated waters percolated, and the 

 chemical rearrangement proceeded outward into fragments of breccia. 

 To this process the banding must be ascribed, but it frequently repro- 

 duces the efi'ect of a succession of layers deposited in a cavity. 



Analcite 



In the Watchung series, analcite was one of the first of the zeolites to 

 appear, but under exceptional circumstances was apparently preceded by 

 others. In the slides, it appears quite frequently, and conclusions regard- 

 ing its sequence have been supplemented by macroscopic observations. 



When fresh, it is perfectly isotropic and, where free from inclusions, is 

 water-white, but with incipient decomposition it becomes very muddy. 

 Further advance produces a porous mass, riddled with cavities. Under 

 the latter conditions, it loses its isotropic character and becomes notice- 

 ably birefringent. Crystal outlines are sometimes lacking or very doubt- 

 ful. In the fresher specimens, there appears to be a slight indication of 

 cubic cleavage, but the fracture is in general conchoidal or follows irregu- 

 lar lines. Index of refraction less than balsam. Chabazite resembles 

 analcite in its low refraction and low double refraction, but chabazite has 

 such very low refraction that the efEect of high relief is given. In addi- 

 tion, it has a form of blocky cleavage which is characteristic, and, in most 

 cases, there is very little doubt as to which is present. 



Analcite sometimes occurs in broad areas, as in 60 and 61, but is also 

 apt to appear in vermicular bands. Both modes of occurrence are shown 

 in Plate XIII, fig. 3, from slide 3. The bleached bands and cusplike 

 figures which have been spoken of in connection with the manner in 

 which zeolitic alteration attacks the rock are believed to consist largely of 

 analcite, mixed probably with some lime zeolite. The transformation of 

 albite and anorthite molecules of the glass into zeolites was probably 

 effected with the greatest facility on account of the small amount of 

 chemical rearrangement necessary. The first effect of such a change 

 would probably be an intimate intermingling of species, but later, on 

 account of the tendency which large crystals have to grow at the expense 

 of smaller, segregation would occur, and the crystals would be individual- 

 ized. 



