126 ANNALS NEW YORK ACADEMY OF SCIENCES 



tion, but little of it now survives. Intermediate forms between the large 

 and small types described appear in several slides, for instance, 68, 70, 

 119 and 120, generally in isolated crystals or small groups, and show 

 active replacement. 



A frequent mode of occurrence of albite in these rocks is as part of the 

 filling of small, sharply defined veins which often cut the walls of quar- 

 ries. They have a northeasterly and southwesterly strike. The veins 

 bear evidence of being the result of deformative stresses which have 

 affected the region. The presence of albite, which was probably one of the 

 first products of recrystallization and which was formed while the rocks 

 were still in a superheated condition, indicates probably that the regional 

 adjustments by which the cracks were formed took place within a com- 

 paratively short period after the extrusion of the lava. 



Replacement of Albite 



A normal association of albite with fibrous amphibole. garnet and 

 specularite, in which the relations are but little obscured by further 

 stages of replacement, appears in slide 98 (Plate XIIT, fig. 1). In this 

 slide a normally crystalline basalt is crusted with a variety of secondary 

 minerals, which in places show evidence in their structure of having 

 replaced a glassy crust of lava. The primary alteration seems to have 

 resulted in albite, green amphibole, garnet and specularite, which appear 

 in close association next to the unaltered basalt. Further out, these 

 products are giving way to prehnite, and at one place sho-mi in the sketch, 

 chabazite is the replacing mineral. 



The typical manner in which replacement attacks the albite is shown 

 in Plate XI, fig. 3, from slide 63. In this slide, finely crystalline basalt 

 is cut by veinlets (probably originating as shrinkage-cracks in cooling) 

 whose filling consists principally of albite and prehnite. In the more 

 minute veins, where there is little prehnite, the albite looks perfectly 

 fresh. The crusts, between crossed nicols, have the appearance shown in 

 Plate XI, fig. 2, the albite being finely granular next to the basalt and 

 assuming a development of larger crystals at a short distance. In the 

 more open areas, where prehnite is abundant, the albite crystals have 

 assumed a spongy appearance (suggestive of ice which has been exposed 

 to the sun) and have lost their sharpness of outline. Fanlike groups of 

 prehnite appear to spring up at any point within the massed crystals of 

 albite and gradually encroach upon them, as shown in Plate XI, fig. 3. 

 The albite in association then appears corroded at thfe edges and turbid 

 within. By a continuation of the invasion, albite has disappeared over 



