52 The American Geologist. juiy, 1892 
with abundant large (4 to 8 mm.) phenocrysts of augite, olivine 
and plagioclase, the olivines l)eing most abundant. Like the true 
camptonites it shows under the microscope a dense groundmass 
of minute brown hornblendes, augites, plagioclase and iron ores, 
all in idiomorphic forms, throughout which are scattered the 
phenocrysts of olivine, augite, and plagioclase above noted. 
A search for similar dikes elsewhere in the vicinity resulted as 
follows :— 
At the Cedar street crossing of the lower branch of the M. C. 
R. R. — a few squares above the mill and to the northward — 
(locality (2) on sketch map) a dike 5 feet in width was found, 
consisting of a compact dull dark gray rock with an abundant 
sprinkling of small olivines. Under the microscope the mineral 
nature of the rock (28523) was found to be quite similar to the 
others mentioned, exceptmg that augites prevailed in excess of 
hornblendes in the groundmass. The structure was also quite 
n^ariable in different sections, in some cases being decidedly por- 
phyritic, as in the camptonites, or again ophitic as in normal dia- 
base. An abundance of secondary calcite here, as in the other 
dikes, obscures everything. There is no doubt l)ut that this be- 
longs to the same series of dikes. Still further northward, just 
below the Maine Central railroad bridge over the Androscoggin 
jiver, and running nearly parallel with the bridge is a dike some 
five feet in width of a dense dark greenish rock (28534) which 
under the microscope shows the ophitic structure of ordinary dia- 
base, and which yielded Prof. Packard 49.75% Si O^. This dike 
is covered by overlying rocks and soils on either shore and may 
be seen to advantage only in the river bed and at low water. 
(Locality (3) on map). This is presumably the dike mentioned by 
Jackson* as having a direction of N. 80 W. , S. 80 E. , though the 
•>vidth as given by him is but "1^ feet. 
Going southward on the Auburn side of the river the first dike 
(encountered was on the property of L. Merrill, Lake street, above 
Ji'ern. (Locality (4) on maj)). This was in granite and but three 
inches in width. The rock is dark gray, strongly banded parallel 
with the sides of the dike in gray and black colors and showing in 
-the hand specimen simply a compact aphanitic ground without 
crystalline secretions of such size as to be determinable by the 
unaided eye or pocket lens. Under the microscope in thin sec- 
' *Geol. of Maine, 3d Ann. Rep., 1839, p. 19L 
