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ANNALS NEW YORK ACADEMY OF SCIENCES 



These cases were all found in the two eastern areas, although less pro- 

 nounced ones occur in the Montrose Point district. All of these rocks 

 would be classed by Williams as augite peridotites (picrites) which he 

 thought occurred in the highest development on Montrose Point. He 

 describes at considerable length another group from Stony Point, how- 

 ever, which is. not typically represented in the main area — the hornblende 

 peridotites. The nearest approach to these rocks is to be found in the 

 hornblende pyroxenite area on the south side of Montrose Point; but 

 these contain olivine to the extent of more than one-third only exception- 

 ally. Oddly enough, however, Williams, who, after all, studied only a 

 small part of the whole area, felt that they were so important a member 

 of the Cortlandt Series that he proposed the name "Co^tlandtite , ' for 



them, and rocks consisting of olivine 

 and hornblende are now widely 

 known by this name. 



A rather extraordinary rock, some- 

 what analogous to the so-called cort- 

 landite, does, however, occur in a 

 small patch on the extreme eastern 

 edge of the series. This in the hand 

 specimen exhibits the poikilitic 

 structure par excellence, but biotite,. 

 instead of hornblende, is the host. 

 This occurs in individuals often sev- 

 eral inches across (although by rea- 

 son of the abundance of the included 

 minerals, it makes up somewhat less 

 than one-third of the rock), and under the microscope it is extraor- 

 dinary for the abundance of its inclusions. These are of a sharp, 

 brownish black substance, probably magnetite, and are so abundant in 

 the center of the grain as to render it opaque, but the periphery and a 

 margin around each included olivine is entirely clear. On the extreme 

 edge of tbe biotite in the latter case, however, there is a narrow band of 

 magnetite grains, and surrounding, or nearly surrounding, the olivine is 

 a wider zone of pale green pyroxene in rectangular blocks (fig. 1). This 

 appears to be a reaction rim, although the pyroxene zone is not entirely 

 universal. Basaltic hornblende, without inclusions, is fairly abundant, 

 and hypersthene and enstatite make up nearly one-third of the rock. 

 The olivine is also present in about this amount; white angite is almost 

 lacking. Alteration is apparent, but is not severe. 



As the culmination of the basic development along this line, we have 



Fig. 1. Poikilitic Relation of Biotite 

 and Olivine in Peridotite. SI. 166 



