256 PROCEEDINGS OF THE AMERICAX ACADEMY. 



ent and is often so abundant as to be megascopically visible. These 

 structures seem to the writer to be explained by the hypothesis that 

 they were produced by the breaking and tearing off of the first con- 

 solidated portions of the porphyry magma, which had formed against 

 the cooler rocks of the contact zone, by movements in the still partially 

 fluid, and doubtless highly viscous, mass beneath. With these were 

 doubtless fragments of the aporhyolite not now clearly distinguishable. 

 Once included, the fragments were modified in shape and more or less 

 recrystallized by the hot, enclosing mass, with its mineralizing vapors. 

 The porphyry of the contact zone may, therefore, as a whole, be 

 regarded as possessing a somewhat modified taxitic structure. 



Contact Porphyry of the Pine Hill Area. — The contact phase of the 

 porphyry as developed on the eastern slopes of Pine Hill are deserving 

 of special notice (see here special map of Pine Hill area). In the first 

 place the distance from the granite to the felsite aporhyolite as has 

 been noted earlier, is much narrower than elsewhere, ranging from 

 perhaps 15 ft. at the extreme eastern end to perhaps 30 ft. a little east 

 of the summit of the hill. The narrower contact zone indicates that 

 here, as held by Professor Crosby, a deep part of the contact zone is 

 exposed and the characters of the rocks fully bear this out. At the 

 immediate contact with the aporhyolite we find a very dense, not 

 conspicuously porphyritic rock. In fact, it is often impossible without 

 microscopic preparations to tell when one is dealing with the aporhyo- 

 lite and when with the porphyry. The similarity is increased by the 

 fact that the porphyry at the contact, here as elsewhere, is much 

 brecciated, giving rise not only to the appearance of flow structure 

 but also to that of small apophyses of dense material running into the 

 porphyry, and these might easily be mistaken for apophyses of the 

 aporhyolite cutting the porphyry. Such they were supposed to be by 

 Crosby and they naturally constituted the strongest argument for 

 the intrusive nature of the aporhyolite.^'^ 



A few inches back from the contact, the rock quite suddenly as- 

 sumes a strongly and coarsely porphyritic habit. The quartz and 

 feldspar phenocrysts here attain the largest size met an;^^'here in the 

 field and the feldspar is relatively more abundant than elsewhere. 

 (See table, p. 243, Column IV, of Rosival measurements on the 



3 7 To illustrate how natural was this mistake regarding the nature of this con- 

 tact the writer may say that he collected several suites of specimens ilhistrating 

 as he supposed the succession of tj^pes across the contact, only to find, when 

 they were sectioned, that they were all of the porphyry and that the aporhyo- 

 lite had not; been reached. The true contact was found two or three inches 

 beyond. 



