WARREN. — ALKALI-GRANITES AND PORPHYRIES. 315 



nected in the field with the rhombenporphyry. That all of the 

 xenoliths were formed in this way is by no means so certain. 



So far as the strongly porphyritic xenoliths are concerned it is 

 important to note that they all exhibit the same characteristics as the 

 granite-porphyry and the rhombenporphyry, in that they have pheno- 

 crysts of feldspar, usually of more or less distinct " rhomben " habit, 

 which show a pause in the progress of crystallization followed by a 

 simultaneous crystallization of the groundmass minerals. Further- 

 more the feldspar phenocrysts in the great majority of cases, and always 

 in the xenoliths of the " rhomben " type of phenocrysts, are, or show 

 positive evidence of having been, either a homogeneous soda-potash 

 feldspar or very fine cryptoperthite, viz. — the same feldspar which 

 is characteristic of the quickly cooled (quenched) phases of the magma. 

 It seems improbable that we should find a small mass of rock possess- 

 ing a texture indicati^■e of relatively rapid cooling and containing 

 phenocrysts of a type of feldspar, which elsewhere in the field formed 

 only under conditions of rapid cooling, developing such peculiarities in 

 situ enclosed in a rock which itself is typically granitoid and developed 

 under conditions of slow cooling. It might be contended that these 

 xenoliths have developed about one or more crystals of the feldspar, 

 acting as nuclei, by some process of fractional crystallization, and 

 that the rim of later growth was due to a growth of larger crystals at 

 the expense of smaller ones during the period in which the material 

 was surrounded by the hot, granite magma. The presence of poikili- 

 tically developed hornl)lende perhaps might also be cited to support 

 this contention. It appears, however, to the writer that in such a 

 case the same action of the hot magma would pretty certainly have 

 effected a recrystallization of the feldspar phenocrysts since, as we 

 have shown, these are very easily changed, and such changes have not 

 been observed to be common in the feldspar phenocrysts of this type 

 of xenolith ; nor does it seem likely that so perfect a reproduction of 

 the normal porphyritic texture could result. Furthermore, a process 

 of gradual accumulation from the surrounding magma could hardly 

 be expected to result here in sharp contacts, which are a character- 

 istic of all of the xenoliths. The rounded, irregular and indented 

 contacts of the xenoliths with the enclosing granite, so beautifully 

 exposed on the glaciated ledge beside the railroad track east of Pine 

 Hill, and the flow structures sometimes observed in them, suggest 

 simply a softening and moulding of the xenolithic masses by the hot 

 granite magma. It might also be contended that they represent con- 

 solidated liquid segregations formed in situ and that their finer tex- 



