98 Camptonite Dikes^ Etc. — Marsters. 
shown by C. D. Walcott', consists of Precambrian strata west 
of lake Champlain, then to the east a band of Hudson 
River slates, then the Trenton, Chazy, &c., limestones, 
and lastly the Georgia slates, which contain the dikes. 
These slaty rocks are considerably distorted and folded, and 
in comparatively short distances vary in dip from 20° to 35.° 
In the vicinity of the dikes they are decidedly calcareous, and 
for considerable distances on either side are very much broken 
and seamed with veins of calcite. As compared with the over- 
lying band of roofing slates to the east, they are somewhat 
softer, and a lighter gray in color. One dike strikes N. 40° E, 
the other N. 80° E. They vary from 3 to 4 feet in width. 
Macroscopically they are dark gray in color, quite soft 
and possess a peculiar granular appearance. They are also 
peculiar in that they exhibit amygdaloidal cavities, a phe- 
nomenon not shown by any other dike rocks of our acquaint- 
ance. The amygdules are filled mainly with calcite and vary 
in size from ^inch to such minute masses as can only be seen 
by the aid of the microscope. Occasional well developed 
crystals of augite are seen scattered through the rock mass. 
Microscopic examination proves these rocks to be made up 
of augite, hornblende, plagioclase and innumerable small 
masses of magnetite, portions of which show complete crys- 
tallographic forms. 
The augite occurs in well formed porphyritic crystals of 
pink and green colors showing prismatic cleavage. A peculiar 
zonal structure was noted in several instances in which the 
nucleus of the augite crystal appeared under ordinary light to 
be light green in color while the outer rim was of a light yel- 
low. This is probably due to a differing chemical composi- 
tion in the outer rim and inner nucleus. Occasionally the 
augite was slightly decomposed, but to a certain degree retained 
its crystallographic form and optical properties. A close ex- 
amination of it shows some additional interesting features. 
Included in the porphyritic augite occur small prisms of 
brown hornblende identical with that to be later described as 
the principal component of the rock. While the vertical axes 
in the two minerals appear to be parallel the others do not 
(1) American Journal of Science, ill series, vol. xxxv, March, 1888. 
We have been much assisted by Mr. Walcott's map, and by notes 
kindly loaned by Dr. J. Francis Williams, of the Pratt Inst., Brooklyn. 
