184 



in New Brunswick, at the opposite end of the Gulf of Maine, 

 and in Newfoundland. 



In determining the relations of the other uncrystallines of the 

 Boston basin to the Braintree beds, we are obliged, on account of 

 the entire absence in the former, so far as known, of recognizable 

 organic remains, to be guided wholly by their lithological and 

 stratigraphical characters. The stratigraphical test is, of course, 

 all-sufficient, where it is possible to command the data necessary 

 for its application ; but the rocks in question have been folded, 

 faulted, denuded, and drift-covered to an extent which makes 

 the structure of the region exceedingly difficult to unravel. The 

 construction of a detailed geological map of this basin, however, 

 has assisted very materially in furnishing a clue to the disposi- 

 tion of the strata below the surface. Although geologists are 

 well agreed that, as a general rule, lithological and mineralog- 

 ical characters are of little value as tests of age among noncrys- 

 talline sediments ; yet every observer knows that where the area 

 considered is not great, this sort of evidence often leads to con- 

 clusions of the highest certainty, the rocks not infrequently pos- 

 sessing peculiar characters by which they may be unfailingly 

 identified ; and, as a matter of fact, it is in this way, more often 

 than in any other, that formations are traced and mapped. The 

 student neither needs nor expects to find the characteristic fos- 

 sils in every outcrop. The true horizon of a set of beds having 

 been definitely determined by paleontologic or stratigraphic 

 evidence at one or more points in a given district, it is then 

 perfectly legitimate to note the distinctive features — either of 

 composition or structure — of the beds, and proceed to trace 

 their distribution by reference to these marks alone. 



The Paradoxides bed in Braintree is, of course, the estab- 

 lished base line for the stratigraphy of the Boston basin ; and, 

 proceeding in the manner indicated above, supplementing the 

 imperfect stratigraphical evidence by lithological data, I have 

 endeavored to ascertain the positions with reference to this known 

 horizon of the other rocks of this basin. The general result 

 reached has already been stated ; viz., that all the rocks in the 



