72 
IOWA ACADEMY OF SCIENCES. 
NOTES ON THE GEOLOGY OF THE BOSTON BASIN. 
J. L. TILTON. 
The region about Boston forms a basin. Standing on the 
reservoir at College Hill one looks north, west and south upon 
lines of hills surrounding Boston and the thickly populated 
adjoining country. In the relation of the rocks underlying the 
drifts this region also forms a basin. The distant hills are of 
hornblende granite extending from near Marblehead southwest 
to near south Natick, thence east toward Qaincy. Close to this 
granite area are other igneous rocks, and within the basin, con- 
glomerate and slate so related and concealed by drift as to 
present many difficult problems. 
It is not surprising that the discussion* of the area contains 
not only a mass of conflicting conclusions, but even a mass of 
conflicting statements concerning field evidence. The rocks 
seemed to grade into one another; the felsite along the margin 
of the basin appeared where observed to penetrate the granite 
instead of the granite the felsite; the flow structure seemed 
Stratification; the sedimentary material is so related to the 
igneous rock and presents plains of stratification so obscure 
and nearly vertical that to some the conglomerate appeared 
uppermost, to others the slate uppermost, while to still another 
■there seemed to be two beds of conglomerate. For years it 
■was agreed that the felsite, porphory and diorite were all 
originally sediments changed to their present conditions by 
varying degrees of metamorphism. 
In age the sedimentary rocks were variously classified, Cam- 
brian, Devonion or Carboniferous. 
Since 1877, Dr. M. E. Wadsworth and Mr. J. S. Diller have 
given careful attention to these problems. In conclusion Mr. 
Biller, t after a presentation of evidence that seems incontro- 
*The discussion is given in full in “The Azoic System,” Whitney and Wadsworth, 
Bull. Mus. Comp. Zool. at Cambridge, Mass., Vol. VII. 
+ “Felsites and their Associated Rocks north of Boston,” J. S. Diller, Bull. Mus. 
C/omp. Zool. at Cambridge, Mass., Vol. VII. 
