38 History of H Ingham. 



being left intact upon one of the sides of the joint, as would have 

 been judged likely, whatever the force that rent the rock apart. 



When two series of the joints under consideration are observa- 

 ble upon any rock surface, those of one series running in a cer- 

 tain direction will be found often to be intersected by those of the 

 other, the result being to separate the rock more or less vertically 

 in rectangular or rhomboidal divisions, and when, as is often the 

 case in slates, there are also joints of expansion, cleavage planes, 

 or planes of stratification, which are more or less transverse to the 

 two mentioned, the rock will break into rectangular or rhomboidal 

 blocks. 



Such may be obtained at Huit's Cove or more readily at Slate 

 Island just outside the town limits. Fine specimens may be seen 

 in the town collection of rocks from the former locality. 



It remains now to state the probable origin of the vertical in- 

 tersecting joints. Much study has been given to the subject by 

 several o-eolos;ists. To Professor W. 0. Crosbv is certainly due 

 the credit of susro-estini:; and ablv advocating a theory that seems 

 to the writer after much consideration, conclusive. The theory 

 is that eartbquake action caused the phenomena. Space will not 

 here allow further remarks, but the reader who wishes to learn 

 more of it, is referred to the Proceedings of the Boston Society of 

 Natural History, vols. xxii. and xxiii. 



THE BOSTON BASIN. 



The border line of the granitic and dioritic rocks of the town, 

 whether near or far from the coast and however irregular its 

 course, may be regarded approximately as part of the border of a 

 great area which is known by geologists as the Boston Basin, and 

 which embraces a portion of the towns of Cohassct, Hull, Hing- 

 ham, Weymouth, Quincy, Milton, Hyde Park, Needham, Newton, 

 Brookline, Somerville, Cambridge, Watertown, Maiden, Medford, 

 Everett, and the city of Boston, with its harbor east to the outer 

 islands, and possibly a considerable distance beyond, the diameter 

 east-west being not far from twenty-five miles, and having a north- 

 south diameter averaging about twelve miles. 



It is absolutely necessary to know much of the history of the 

 formations of the Boston Basin in order to appreciate what may 

 be said of that portion embraced within the limits of Hingham. 

 There has been much discussion carried on over a long period 

 respecting the age and the sequence of its formations. Recent 

 investigations in all parts of it by Professor Crosby have thrown 

 much light upon the subject, changing materially his own views 

 and those of others, who have been informed of his important 

 observations. 



One result of his work has been to establish the fact, that in- 

 stead of there beino: but one formation of slate, as advocated by 

 himself, there is shown clearly to be two, as claimed by other oh- 



