308 PROCEEDINGS OF THE AMERICAN ACADEMY. 



found on North Common Hill, which seem to represent the deepest 

 contacts an\"vvhere exposed and which have been brought up to -their 

 present relatively high positions by faulting and elevation at the north, 

 no rhombenporphyry is now found at the contact, whereas if it is true, 

 as had been assumed, that it is in places where the magma i*emained 

 hot for a long period, thus retaining its fluidity and permitting differ- 

 entiation to take place, it is precisely here that the products of differ- 

 entiation should be found. In view of the fact that elsewhere in the 

 field the first formed consolidations have generally been broken and 

 torn away from their original position at the contacts, it is to be 

 expected that at deeper contacts where the magma was not chilled 

 and retained its mobility longer, any differentiation products that 

 might form, would sink or be otherwise moved from their original 

 place of formation and dispersed through magma. Indeed, the 

 numerous patches of ^•arying texture and more basic composition, 

 found everywhere throughout the granite, are believed to be evidence 

 that such action has in fact taken place. It does not, however, appear 

 necessary to assume that differentiation would occur at very deep 

 contacts in the same manner that it would at higher elevations, where 

 the temperature gradient would be much greater where other condi- 

 tions would likewise be different. 



In places, as in the neighborhood of the contacts in Northern Pine 

 Hill, abundant xenoliths of what are quite obviously fragments of the 

 contact phases are very abundant, ha\ing been caught in the freezing 

 liquid before they had had time to move, or be moved, far from their 

 original positions. The general distribution of the contact phases 

 through that part of the magma which in time formed the granite is 

 held to account for the presence of the xenoliths generally. In this 

 connection it should be noted that the irregular variation in the com- 

 position of the granite, as shown by the microscope and the chemical 

 analyses, is suggestive of movements in a magma not perfectly homo- 

 geneous in composition, and this is in keeping with the other evidences 

 above set forth of more or less strong movement in the intruding 

 magma before complete solidification took place. As further bearing 

 on the question of movement, we should note the strong testimony of 

 the broken phenocrysts of the granite-porphyry and of the proto- 

 clastic structures in certain portions of the granite (that of the Gold- 

 leaf Quarry). 



Order of CrysiaUization in the various yhascs of the Magma. — The 

 crystallization of the various rock types has been purposely left until 

 they could all be discussed together. The relations of the various 



