Origin of Plicnocrysts. — Crosby. 301 
ing the cooling of the magma or escape of water diminishes 
the crystallization interval; hence the minerals of the first in- 
tervals (phenocrysts) are coarser than those of the later inter- 
vals (groundmass). The porphyritic structure is in this case a 
sort of arrested or retarded granular structure ; and the phen- 
ocrysts belong to the monogenetic type — distinct from the 
groundmass. Phenocrysts are, however, usually of the re- 
current type, agreeing mineralogically with a particular con- 
stituent of the groundmass; and these are believed to depend 
upon mass action, each compound present in predominant 
amount tending ta set up centers of crystallization before its 
proper period. Recurrent phenocrysts thus belong to the 
more abundant species in the rock, as is commonly found to be 
the case. 
These explanations, alike of the monogenetic and the re- 
current phenocrysts, appear to the writer to still leave some- 
thing to be desired in the way of fully and satisfactorily ac- 
counting for the usually very striking contrast of phenocrysts 
and groundmass. The following statement of my own views, 
so far as applicable to the quartz porphyry of the Blue hills, is 
copied almost verbatim from letters written in February and 
March, 1898. 
The quartz porphyry of the Blue hills is not intrusive, ex- 
cept to a very limited extent, where it forms slender apophyses 
in the Cambrian slates ; and these apophyses are macrosropi- 
cally indistinguishable from the main body of the porphyry. 
The quartz porphyry is as massive and free from flow-struc- 
ture as the granite which it covers, simply forming the con- 
tact zone of the batholith between the granite and the Cam- 
brian strata, and passing insensibly downward through gran- 
ite porphyry into the normal granite in which the quartz is 
wholly allotriomorphic. Even at the contact with the slates, 
the porphyry holds its normal character, and is not appreciably 
more compact (felsitic) or fluidal. 
The floor on which the Cambrian strata were deposited has 
utterly disappeared during the development of the batholith ; 
and the most rational conclusion seems to be that the magma, 
due itself to a great heat invasion consequent, probably, upon 
extensive deposition and subsequent deformation, absorbed by 
melting or solution not only the whole of the sub-Cambrian 
