Crosby.] 436 [October 17, 



such as a violent earthquake, the wedge-shaped mass of rock in- 

 cluded between two master joints converging downwards has 

 dropped, while the rocks were in a state of tension, to a depth of 

 one hundred feet, more or less, thus giving rise to the chasm. 



This explanation is in harmony with all the facts observed. 

 The chasm, as previously stated, is parallel with a well defined 

 system of joints, and its walls are evidently joint planes. No 

 erosion is required and evidences of erosion are entirely wanting. 

 The angular blocks with which the bottom of the chasm is filled 

 are just such as would naturally be produced by such a catastro- 

 phe as is here conceived. Finally, there can be no doubt that 

 parallel master joints frequently do converge downwards, that 

 during the passage of an earthquake the rocks are thrown alter- 

 nately into states of powerful tension and compression ; and that, 

 since earthquakes are of almost annual occurrence in New Eng- 

 land, we may fairly suppose that they are sometimes sufficiently 

 violent to accomplish results such as are here ascribed. 



The dislocation here appealed to as a cause of the chasm is one 

 that geologists will recognize as what might be called a normal 

 form of geological accident. In Jukes' Manual of Geology it is 

 regarded as the best explanation of normal faults. The fracture 

 bounding each side of the wedge-shaped mass which slips down 

 is necessarily a normal fault. 



The facts observed also furnish a clue to the geological age of 

 the chasm. However formed, it must be of post-glacial origin ; 

 for, lying across the path of the ice-sheet it would, if antedating 

 the glacial epoch, have been scraped full of glacial detritus, 

 whereas it is singularly free from such material. The ice-sheet 

 swept the till from this elevation, leaving it bare and rocky, and 

 then at some subsequent period the chasm was formed as ex- 

 plained. It is a common but evident error to refer all such geo- 

 logical events to some remote period ; and I have seen nothing 

 inconsistent with the view that Purgatory, in Sutton, is, geologi- 

 cally speaking, of very recent origin; being, perhaps, only a few 

 centuries old. 



