49 
ON SOME BASIC ERUPTIVE ROCKS IN THE VICIN- 
ITY OF LEWISTON AND AU BURN, ANDROS- 
COGGIN CO., MAINE. 
Plate II. 
By Geouge p. Merrill, Washington, D. C, with anal.vses by R. L.Packard. 
The prevailing underlying rock in Androscoggin, as in ad- 
jacent portions of Cumberland, Oxford, Kennebec and Sagadahoc 
counties, is a coarse and variable micaceous or hornblendic gneiss 
often including considerable thicknesses of crystalline limestone, 
which is colored on Hitchcock's map of the state as of Montalban 
age. Through this gneiss there has every where been injected a 
coarse highly feldspathic gi-anite which forms the main mass of 
the more prominent hills or mountains of the vicinity. The so- 
called -^David's Mountain" near Bates College in Lewiston is a 
good example of this granite. Contact between the molten granite 
and the more calcareous portions of the gneiss has given rise to 
various secondary' minerals, including the vesuvianite and cinna- 
mon garnets which are found at various points within the coun- 
ties mentioned. Into numerous small fissures in granite and 
gneiss alike have subsequently been intruded basic eruptives, some 
of which are of interest in the present stage of petrographical 
knowledge and none of which have as yet received the attention 
they deserve. 
These dikes are all small, and neither their direction nor inclina- 
tion with the horizon is at all constant. Some of them are nearly 
vertical and run in a nearly east and west direction. Others are 
inclined at a very consideralile angle. The original fractures 
through which the molten magma was protruded seem to have 
been very irregular, owing to the varying character of the rock 
masses traversed, and the material itself was too limited in amount 
to make its influence felt. 
The dikes with which this paper has principally to deal are 
situated to the southward and almost within the cit}' limits of 
Lewiston and Auburn and therefore on both sides of the Andros- 
coggin river. The best exposures are on the Lewiston side iu a 
quarry of limestone on the lower branch of the Maine Central 
railroad some 300 yards south of the Androscoggin mill. 
(Locality (1) on sketch map.)* There is here a considerable bluff 
of limestone facing the river, a part of which has been quarried 
*This is the locality described briefly by Hunt, Chemical and Geologi- 
cal Essays, p. 195. 
