100 Camptonite Bikes, Etc. — Marsters. 
ment previous to the complete solidification of the rock. 
They are highly pleochroic c=b> a. The individual crystals 
vary little in size, the average being about 0.1 m.m. In some 
portions 'of the dike the hornblende has undergone marked 
decomposition, the product being a greenish mineral resem- 
bling chlorite. In one instance, (Stat. M.) every constituent 
had undergone decomposition. Magnetite occurs original 
and secondary. In the former octahedral forms were shown 
included in tt^e porphyritic constituent and varying in size 
from .02 m.m. to .04 m.m. while the latter appears only in strings 
of fibrous aggregations. The plagioclase occurs in small rod- 
shaped crystals in many cases very much decomposed ; where 
fresh ones were noted they did not exceed 0.5 m.m. 
While in the region specimens were also obtained by Mr. 
Marsters from a dike 3 feet in width which cuts the marble 
quarries two miles south of Proctor ' on the Rutland and Bur- 
lington R. R., some 25 miles northeast of the dikes mentioned 
above. The macroscopic character is similar and the miner- 
alogical composition is shown by the microscope to be the 
same in all respects as those described above. Slides of the 
marble adjoining the dike exhibit a very finely granular mass 
of calcite crystals, but no special contact minerals gr phe- 
nomena. 
This marble belongs to the Eolian group of Hitchcock,- but 
is referred to the Chazy, etc., by Walcott. It will be noted 
that it is of later age than the Georgia slates and if this dike 
belongs, as its similar character would suggest, to the same 
time of formation as the others, this would-be subsequent to 
the deposition of the Trenton series.' We are disposed to 
connect them with the general upheaval of the Green moun- 
tains which closed the formation of the Lower Silurian 
system. 
The chemical composition of these dikes is shown by the 
analyses of the accompanying table. By way of comparison 
three other typical Camptonites are appended : (See p. 101.) 
The especial interest of these rocks lies in the fact that they 
establish an additional occurrence of the pure Camptonite type 
, ^( ! 
^ Mr. Marsters is greatly indebted to Mr. Taylor, of the Proctor Mar- 
ble Works for assistance given in finding this dike. 
■' Geol. of Vt. vol. I, pp. 399-397. 
^ Using this and other terms in the sense proposed by the Interna- 
tional GeoL Congress. (See Rept. of Amer. Com., 1886, p. 50). 
