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. Wads worth and Mr. J. S. Diller have 

 given careful attention to these problems. In conclusion Mr. 

 Diller, 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 Oamhridge, Mass., Vol. VII. 



t '-Felsites and their Associated Rocks north of Boston," J. S. Diller, Bull. Mus. 

 Oomp. Zool. at Cambridge, Mass., Vol, VII. 



