162 ANNALS NEW YORK ACADEMY OF SCIENCES 



complications are of an extreme order, and it is only by following the 

 process through various degrees of alteration that it has been possible to 

 arrive at satisfactory conclusions as to the genesis of the forms. 



In slide 54 (Plate XII, fig. 6), the inception of the process is illus- 

 trated. The greater portion of the slide shows a fairly normal glass of a 

 dark olive-green color, with a sprinkling of phenocrysts. Serpentinous 

 or chloritic nodules from resorbed olivine are numerous. The attack by 

 heated waters results characteristically in the appearance shown in the 

 figure. The lighter bands represent lines of replacement, which often 

 assume vermicular or crescentic forms. The course of the waters was in 

 many cases guided by chill cracks which often show distinctly along the 

 middle of bleached bands. Phenocrysts and spherulites have had an 

 influence in the replacement, and there are indications that differences 

 of composition or of physical state along flow lines, and the form ol 

 fragments of partly reabsorbed breccia were not without effect. The 

 change consisted of a chemical rearrangement by which the soda-lime- 

 alumina silicates passed into zeolites, and the ferromagnesian constit- 

 uents were altered to chloritic aggregates or, less commonly, to serpentine. 



At the point sketched, the zeolite appears perfectly isotropic and 

 almost structureless. In other parts of the slide, similar bands show a 

 feeble but unmistakable polarization. Analcite, chabazite and heu- 

 landite are suggested as possibilities, but specific identification is not 

 attainable. 



In 30 (Plate XII, fig. 2), the process has reached a slightly more ad- 

 vanced stage. In both this illustration and in Plate XII, fig. 6, a struc- 

 ture may be seen which is very characteristic of these incipient stages of 

 alteration, that is, a sort of "stream effect" from the darker to the lighter 

 areas. This may possibly be similar in origin to the stream lines seen 

 when crystals of a soluble salt are placed in water, and is accentuated by 

 the first stages of alteration. In other portions of the slides, the transi- 

 tions from the bleached into the darker areas are by almost imperceptible 

 gradations. 



Much of the original diopside looks fresh, but it is plainly breaking 

 up into separate granules by chloritic alteration along cracks. The small 

 plagioclase laths retain their form, but hare altered into some material 

 giving aggregate polarization. 



Slide 3 was prepared from a specimen of glass which appears to have 

 been shattered in cooling. Along the cracks thus opened up, ferruginous 

 Band from external sources had been deposited. Later the heated solu- 

 tions followed the same lines. The resulting alteration has taken a 

 similar course to that described for 54 and 30, but it has advanced far- 



