185 



Boston basin, above the Shawmut group, and including the 

 Paradoxides bed, belong to one and the same essentially con- 

 formable series. Although contravening the recorded opinions 

 of many competent observers, I experience but little hesitation 

 in announcing this important correlation. The evidence upon 

 which it is based will be found in the general account of the 

 stratigraphy of the basin which follows. 



The uncrystalline sediments in the vicinity of Boston are com- 

 monly classified as conglomerate, or " Roxbury pudding-stone," 

 and slate. The existence in this district of sandstone, grit, and 

 limestone, is also well known ; but still these are of very sub- 

 ordinate importance. The conglomerate and slate occur prin- 

 cipally in large masses, each rock covering tracts some of which 

 are ten to twenty square miles in extent. All the areas of these 

 rocks are elongated in an E.N.E. and W. S.W. direction, 

 parallel with the general line of strike and the axis of the 

 basin ; so that a north-south traverse shows several alterna- 

 tions of belts of conglomerate and slate. The general sepa- 

 rateness of these two rocks is an important and well-established 

 fact ; for, although beds of fine material, sandstone, and even 

 slate, are of common occurrence in the conglomerate, yet they 

 are essentially a part of that rock, are always of limited extent, 

 and clearly to be distinguished from the great mass of the slate. 

 These intercalated layers, however, are of considerable assistance 

 to the observer, shice without them it would rarely be possible to 

 determine the strike and dip of the conglomerate. They have, 

 of course, been ignored on the map, and this circumstance 

 helps to a still better appreciation of the fact that the true con- 

 glomerate and slate are rarely interstratified in any detailed 

 manner. 



Nevertheless, the precise stratigraphic relations of the con- 

 glomerate and slate have long been a vexed question with 

 the students of our local geology. Observers are divided on 

 the question as to whether the slate underlies the conglom- 

 erate, or vice versa; while some have supposed that there are 

 two slate formations, with- the conglomerate between ; and others 



