Dike* Xcur Lyon J/7., 2V. T, — Eakle. 33 
orthoclase are commonly twinned and more or less kaolinized. 
A very small amount of plagioclase appears in the usual 
lath-shaped rods. As is seen by the analysis of this dike, 
the amount of soda is quite high and suggests anorthoclase 
as part of the groundmass, owing to the plagioclase being 
insufficient in quantity to account for it. Irregular grains of 
quartz occur as an interstitial filling. Chlorite, as an altera- 
tion product of augite, is common. In a few crystals the 
core remains sufficiently unaltered to permit of its identifica- 
tion as augite. Magnetite docs not appear in the slide but 
hematite is common as a stain. 
The name "bOstonite"* has been applied to certain porphy- 
ries and trachytes, occurring especially with elseolite syenites, 
and professors Kemp and Marsters have adopted flu- name 
for the porphyries of lake Champlain. These porphyries are 
much lighter in color and more porphyritic than dike 1 : 
their color in general is quite similar to that of the keratophyr 
described by Searsf. The main difference, however, lies in 
the presence of much chloritized augite in dike 1, while in 
the bostonites of lake Champlain no dark silicates occur. In 
the two cases, the widths of the dikes and the nature of the 
enclosing rock are quite different. From the writer's knowl- 
edge of the lake Champlain porphyries, they arc generally 
wide masses intruding through the shales of the [Jtica and 
Hudson River stages, while dike 1 is only one foot wide and 
enclosed by solid walls of gneiss. Possibly the different con- 
ditions of environment may have modified the character of 
the rock constituents. 
The amount of silica in dike 1 is higher than in either the 
bostonite or keratophyr. The amount of magnesia is smaller 
than one would naturally suppose, from the amount of green 
chlorite in the slide. In its composition the dyke is closely 
allied to the keratophyr, but in appearance it is quite 
different. 
Dikes 2 and ;$, and those inland, are of the basic type and 
do not differ greatly from the common olivine diabases of the 
region. They are dark, almost black, and very compact. 
*Hunter, M., and Rosenbusch, H. Tdchermak's Min. u. Petr. Mitth., 
xi, p. 445, 1890. 
fSears, J. H. On Keratophyr from Marblehead Neck. Bull. Mus. 
Comp. Zonl., xvi, 9-170. 
