1891. | NEW YORK ACADEMY OF SCIENCES. 19 
lacking feldspar entirely, and having comparatively little glass. 
Olivine is occasionally seen in a fresh condition, but in most dikes 
an alteration product not always satisfactory alone indicates its 
probable original presence. The basic dikes are described under 
the following three types, diabase including olivine-diabase, camp- 
tonite, and moncliquite. 
The Diabase. 
The true diabase dikes are principally, if not quite entirely found 
in the crystalline Archean areas. They are thus on the west side 
of the lake, and in several cases back in the mountains. Diabase 
is the characteristic type of rock that forms the dikes which so 
plentifully intersect the lenses of magnetite. 
The rocks exhibit, under the microscope, the characteristic ophitic 
structure with occasional radiating arrangement of feldspars 
(divergent-strahlig of Lossen). These have, at times, a dark core 
which follows the outline of the crystal, and is, doubtless, an 
altered portion stained by infiltration. The plagioclase was sepa- 
rated from a coarse dike from near Port Kent, and afforded the fol- 
lowing analysis :— 
re II. At. Ratio. 
Loss 0.72 
SiO, 57.82 58.38 973 il 
Al,0, 28.16 28.43 .276 3 
CaO 7.12 fet) .139 2 
Na,O 5.35 5.40 .O87 I 
99.77 100.00 
From this it appears that the feldspar belongs in the andesite 
series. Column II is recalculated to throw out the loss on ignition. 
The augite tends to become at times idiomorphic, and marks a 
passage into the augite-camptonites. This tendency is wide-spread, 
and indicates the close affinities of the two rocks. The augite 
varies from pink to greenish. In the heavy residues obtained by 
panning crushed material, some grains of hypersthene were found, 
although none were ever noted in the slides. The discovery is 
interesting in connection with its recently found presence in Triassic 
diabase.t Subordinate biotite is occasionally seen in scales with 
magnetite, and is regarded as an original mineral.? Irregular 
grains of magnetite are abundant. 
The process of alteration seems to be the same wherever noted. 
The bisilicates pass into chlorite, or into what in other cases appears 
to be serpentine. The latter is a yellow or amber colored mass, 
not always showing a very marked, aggregate character. It is 
lacking in pleochroism and is not very strongly refractive. Some 
dikes are in such an advanced stage of alteration that they present 
1 Campbell and Brown, Composition of certain Mesozoic Igneous Rocks from 
Virginia, Bull. Geol. Soc. of America, 1891, p. 339. 
2? Wadsworth takes a different view of similar biotite, regarding it as sec- 
ondary, Bull. ii, Min. Geol. Surv. p. 65, and Pl. iii. 
