140 ANNALS NEW YORK ACADEMY OF SCIENCES 



specimen as 53, and, so far as the relations of garnet are concerned, is 

 almost a duplicate (Plate XIII, fig. 2). 



In 115 and 118, the same relations hold. In several places in 115, the 

 geometrical patterns in which the garnet grains are arranged show up in 

 a remarkable manner. Plate XII, fig. 5, is a sketch of a portion of the 

 slide. With encroachment of prehnite, the figures become obliterated. 

 The resemblance to chloritic nodules derived from resorbed olivine, shown 

 in fig. 2, slide 48, is striking. 



In most of the slides in which garnet has been found in any notable 

 quantity, these circular patterns are a marked feature. The resemblance 

 in form to chlorite nodules is so close that a similar explanation of genesis 

 appears probable. The origin of the chlorite nodules may be traced 

 backward through intermediate steps to phenocrysts of olivine, which, it 

 appears, became unstable during the primary consolidation of the magma 

 and were largely resorbed. Such nuclei of olivinitic material, which had 

 suffered refusion and partial absorption, remained unchanged in the glass 

 after consolidation imtil circulating waters gained access. The time 

 required for this varied according to the relations of the channels of cir- 

 culation, and the nature of the resulting minerals depended upon this 

 period. It appears that when alteration was delayed until a rather late 

 stage, the nodules of resorbed olivine passed over into chlorite, but in an 

 earlier period, while temperature was high, garnet was the chief mineral 

 formed. 



The garnet seems to alter into prehnite with great facility. Botli in 

 the derivation of garnet from olivinitic material and in its transforma- 

 tion into prehnite, the process appears to have followed the usual line 

 taken in these rocks, by which magnesia and iron were reduced. 



In several other slides (for example, 50, 95, 100), small grains of 

 similar characteristics (isotropic and of high relief and hexagonal out- 

 line) are found. Occasionally they form considerable clusters, but fur- 

 ther stages of alteration have supervened and obscured the relations. In 

 slide 50a, Plate XI, fig. 6, clusters of them are seen to form a band in 

 prehnite immediately adjacent and parallel to what appears at first to be 

 unaltered basalt. In the latter, it is found, however, that the feldspars 

 and diopside have been wholly replaced by prehnite, garnet and amphi- 

 bole, while the magnetite has been left almost undisturbed and still out- 

 lines the original texture. 



The AmpJiiboles 



A number of distinctly different varieties of amphibole occur among 

 the secondary minerals. Some of these possess decidedly abnormal char- 



