I)) f nisi res of J^((ke Movphrenifigoii. — M (tvster!<. 29 
unusually fine. The scales of biotito seem to liave adjusted 
themselves in a parallel position as if the mass had been sub- 
jected to great pressure while in a plastic state. In such cases 
the rock presents a somewhat gneissic structure. In a few 
instances the dark colored silicates form l)ut a small part of 
the rock, quartz phenocrysts become insignificant, and the 
crystalline texture is less i)roniinent so that the rock mass at 
times resembles quite closely the bostonites* of lake Cham- 
plain. The rock ordinarily presents all the phases of a true 
granite and is regarded as such in this paper. 
La }iiproj>]nire Dikes. 
There were found associated with the granitic rocks, as well 
as in other parts of the lake, dikes differing quite widely from 
the former, both in phj^sical structure and mineral composi- 
tion. Microscopic examination shows that they fall naturally 
into the ]amproj)hyric group of Rosenbusch. They differ from 
the granites in being dark grey to black in color, with mod- 
cratel}^ fine crj^stalline texture and occasional phenocrysts of 
dark colored silicates, sufficiently well developed to be macro- 
scopicall}' determined. In some instances the phenocrysts 
were found to be jjyroxene ; in others the idiomorphic con- 
stituent proved to be hornblende; while in still other instances 
olivine played the same role. A number of instances were 
noted in which the olivine reached a diameter of one and a 
half inches. It is a significant fact that the olivine pheno- 
crysts were invariably located in the central part of the in- 
trusive mass. On the contact the dikes were generally fine- 
grained and in some instances even flinty, the phenocrysts 
having entirely disappeared. 
Many of the olivine-bearing rocks have suffered decomi)o- 
sition. 'J'he olivine has passetl into serpentine, thus imparting 
a peculiar green tinge to the rock. In some cases a cross 
section of the olivine crystals exhibit a central core of the 
original mineral surrounded by a coating or shell of serj)en- 
tine. All stages of decomposition were found in the thin 
sections. Nf) feldspar was macroscopicall}^ determined except 
in one or two undouV)ted diabases, which were evidentlj'^ very 
*Hiill. No. 107, U. S. G. S.. Trap Dikes of Lake Champlain, by J. F. 
I\t'nip jiiul V. F. Marstcrs. 
