Tilton.] 500 f-^P"! '7, 
OX THE SOUTHWESTERX PART OF THE BOSTON 
BASIX.i 
BY J. L. TILTOX. 
By Boston Basin is here meant that area of rocks of sedi- 
mentary origin that hes near Boston ni a generally low surface 
surrounded by higher hills of granite and other igneous rocks. 
In the fall of 1893 Mr. A. J. Collier found indications that the 
"Basin" was not closed at the sharp turn southwest of South 
Xatick as indicated in Professor Crosby's map of the region, but 
from lack of time was unable to form any definite conclusions. 
Last fall the writer undertook to solve the relation between the 
rocks of aqueous origin with their accompanying igneous rocks 
and the surrounding granite. 
A map of the area between South Natick, South Sherborn, 
and Charles River Village is here presented, on which the more 
important outcrops are marked, and a line di-awn between areas 
of granite and areas of basin rock. Microscope slides have been 
prepared of many outcrops in the region studied, also photo- 
graphs taken of conglomerate and granite exposures; but to 
avoid too o-i-eat detail no descri2:)tions of them are here included. 
Bfjef Statement op the Boundary Line between Basin- 
Area AND Granite. 
The line here drawn traces the outline of the granite area. 
It passes southwest from South Xatick around Morseville, south- 
ward along the western side of Farm Pond, returning north- 
ward near Charles River, though a mile and a half above South 
Xatick it projects westward a short distance. At South Xatick 
it tvu-ns eastward and then southward, forming a large cm-ve 
aroimd Pegan Hill. It then turns eastward again, passing 
south of Dover toward Charles River Village. 
1 The wi-iter wishe.s to acknowledge the assistance of 3Ir. L. S. (iiiswoUl of Har- 
vard University, iin<ler whose direction the field work necesskry to this paper was 
undertaken ; also the assistance of Prof. J. E. Wolff and Mr. C. L. AVhittle, to whose 
examination the microscope slides have l)een subjected. 
