264 



silex forming the shore at this point. Interposed between the 

 petrosilex and sandstone, and forming the base of the latter, is 

 a thin stratum of conglomerate, composed of pebbles of the 

 former rock. On the bank above, many fragments of the 

 sandstone are scattered through the soil ; and there are indica- 

 tions of its occurrence at other points on the Neck. 



It is unquestionably hazardous to connect this sandstone with 

 the other uncrystallines of the Boston basin, and I have done 

 so only provisionally, and with hesitation. Lithological evi- 

 dence of synchronism is almost entirely wanting, and strati- 

 graphical there is none, save that the relations to the underlying 

 crystallines are clearly the same in both cases. Neither the 

 sandstone, nor the pebbly layer underlying it, bear any 

 resemblance to the petrosilex breccia occurring so abundantly 

 in the Marblehead region ; but the conglomerate is much less 

 firm, and in every way newer looking than any of the true 

 breccia. The present position of the sandstone, its entirely 

 undisturbed condition, and its occurrence in the form of loose 

 masses in all parts of the Neck, indicate that it probably 

 underlies Marblehead Harbor and the bar at the south-west 

 end. And perhaps the numerous pebbles of precisely the same 

 sandstone found on the beach on the east side of the Neck 

 may be taken as an indication of a submarine deposit of this 

 rock in that direction. 



'I have elsewhere^ remarked upon the incontrovertible proof 

 which this rock affords of the great antiquity of the depression 

 in which it lies ; for obviously Marblehead Harbor was exca- 

 vated from the bordering Huronian diorite, petrosilex, and 

 granite, before the deposition of the sandstone now lying 

 unconformably and horizontally upon its crystalline floor. So 

 that although the removal of the sand-rock from the harbor, 

 which it doubtless once filled, may represent comparatively 

 recent denudation ; yet the harbor itself must have had sub- 

 stantially its present form in early Paleozoic times, having 



^ American Naturalist, xi., 585. 



