32 The American (reologisf. July, 1895 
pleochroism atul a still lighter yellow or nearly colorless cen- 
ters with little or no pleochroic tendencies. Eimsof magnet- 
ite and reddish opaque masses generally surround the larger 
individuals and in a few cases seem to be partially absorbed 
by the peripheral area of the phenocryst. The irregular 
grains of the ground-mass have undergone more or less de- 
composition giving rise to secondary products. The grains 
as in the case of the phenocrysts are generall}"^ surrounded by 
magnetite rims or have become opaque and gray in color. 
Olivine is also more prominent than in the typical diabases. 
It is present as large crystals which have suifered greater 
change than its host. Fibrous rims and patches of serpentine 
make up the secondary products derived from the olivine. 
Plagioclase forms the remaining part of the rock. The 
crystals are invariably fresh and lath-shaped, but not large. 
Multiple twinning is very common. An arrangement akin to 
flow-structure, or an adjustment of the individual feldspars, 
in a more or less definite direction, is apparent in a few sec- 
tions. Pockets or cavities filled with calciteare quite numer- 
ous in some sections. In some cases they were lined with a 
fibrous mineral resembling serpentine, thus suggesting that 
the original mineral substance maj'^ have been olivine. On 
account of the prominence of olivine the rock is regarded as 
an olicine didhase. No. 17 is (he liest representative of this 
type. Others were found but are so completely weathered as 
to be unfit for microscopic stud}'. 
Cdiiiptonite. — Associated with augite plagioclase rocks were 
found other mixtures of hornblende, augite, olivine and pla- 
gioclase. In two dikes the olivine and augite disappear, leav- 
ing the hornblende and plagioclase ns the (-hief constituents. 
This combination is regarded as the typical camptonite. The 
hornblende is always of the basaltic type, idiomorphie and in 
two generations. In a few dikes the augite forms an appre- 
ciable part of the rock, and in some instances is more promi- 
nent than the hornblende. (U)mparing these with the carap- 
tonites of lake Champlain, it is evident that the persistent 
association of augite with the basaltic hornblende, together 
with its larger growth and idiomorphie habit, is more promi- 
nent in the lake Memphremagog occurrences. Owing to this 
