MUSEUM OF COMPARATIVE ZOOLOGY. 67 



cepted as proved on less complete evidence than would be reasonably 

 required to establish the existence of a single isolated fracture. In like 

 manner, if the hypothesis of the occurrence of faults enables one to 

 correlate a great number of otherwise apparently disconnected and 

 arbitrary facts, it is therefore to be regarded favorably. Finally, if it 

 lead to the detection of facts not before noticed, and thus gives the 

 power of prophecy, it can be considered as fully established. 



One who goes on the ground as far as this nari*ative has now led him, 

 will, I think, find it already difficult not to lean too strongly towards 

 closing the question in favor of one of the suggested explanations instead 

 of maintaining an open mind in the question. As a working hypothesis, 

 let the faults be admitted, and a simple terminology adopted in accord- 

 ance with the explanation that they offer. Let the mass between two 

 faults be called a block ; or, if small, a chip. Let the thick sheet of 

 trap be called the main sheet, and the others, the anterior and posterior 

 respectively, in accordance with their position relative to the main sheet. 

 Certain deductive considerations also need attention before going farther 

 in the field ; in our school excursions, these were briefly discussed on 

 the ridge south of the reservoir, overlooking the Southington plain, 

 during the noon rest of the second day. 



First, if faults are suspected, what must be learned about them before 

 they are completely known 1 The elements of a fault are : its outcrop 

 line, its direction, position, and le'ngth ; the hade of the fiiult plane ; the 

 throw of the fault ; and the width and other characteristics of the fissure. 

 Its depth, its date, and its cause are also subjects for more advanced 

 inquiry. With all these elements in mind as subjects for search, one is 

 more alert to discover them. 



Second, if faults do occur, how can they be best recognized 1 Some 

 easily identified bed is the first requirement, such as the main sheet of 

 trap, which forms prominent ridges in the broad valley between slopes 

 of crystalline rocks east and west, and can be easily seen from a distance. 

 But, besides this, it is important to have also a number of thin hard 

 beds whose lines of outcrops are more sharply defined than those of the 

 broad main sheet, in order to determine the position of the fault with ac- 

 curacy. The anterior and posterior trap sheets serve this purpose nicely. 



Third, if the faults occur, what will be their effect on the topography 

 of a well-denuded monoclinal mass, containing hard and soft beds 1 For 

 the sake of simplicity, the case of a single hard bed reduced nearly to 

 baselevel may be taken alone, and the dip of the monoclinal regarded 

 as constant ; the fault plane will for the time be considei'ed vertical. 



