The Geology of Hingham. 41 



decayed material remains upon them, showing, though but par- 

 tially, the extent of the corrosion, much of the substance having 

 been washed oil' the surface by the denuding action of rains. 



There is certainly no reason to suppose the general condition 

 of the surface of the land prior to the glacial period was different 

 over the area of the early formations of New England from what 

 prevailed over formations of a like age south of glacial action. 



We may therefore picture to ourselves, with good reason, the 

 country everywhere in the neighborhood of Boston covered with 

 hills of considerable altitude, composed of the decayed material 

 of the rocky formations, and having disseminated through it 

 bowlders and pebbles of every size, that had not yet yielded to 

 the decomposing influence. It is well known that corrosive 

 action tends to produce such forms, though of course it is not 

 questioned but that subsequent action of water and attrition had 

 much influence in working a large portion of the pebbles found 

 in the conglomerate into the shapes which they now present. 



The subsidence of the area of the basin after the primordial 

 period mentioned, extending the water surface to the base of hills 

 filled with the material for the conglomerate, the igneous action 

 that followed and was active at times during the formation of 

 that rock, causing more or less of oscillation and change of level 

 to the surface, and the subsequent action of the waves upon the 

 cliffs and beaches of coast margin, together, will amply account 

 for the production of the conglomerate, but it will be recognized 

 that the main factor in such view is to be found in the disintegra- 

 tion of the rocky hills long before the action of other forces. 



The presentation now made of the origin of the conglomerate 

 of the Boston Basin is greatly strengthened by the fact lately 

 called to the notice of the writer by Professor Crosby, — that no 

 pebbles of the basic rock diorite are found in the conglomerate 

 with those of the acidic rocks. All will agree in the statement 

 that pebbles of the granite, the quartzite, and the petrosilex 

 rocks of the northern border of the basin, have contributed 

 largely to make up the conglomerate ; but what became of those 

 of the diorite, a rock quite as abundant in the ancient hills as 

 any of them ? Its absence can only be accounted for by the 

 view that it could not like the others withstand the corrosive 

 action, as did partially the others, and therefore not even pebbles 

 were left to help form the newer rock. Respecting the slates, 

 their origin is clear. Simultaneously with the depression of the 

 area of the basin below the sea level, there would commence a 

 deposit of the finer sediment brought down by the rivers. This 

 may well be thought to have been copious considering the char- 

 acter of the country passed through, everywhere composed of 

 the decayed remains of the earlier rocks. Indeed it cannot be 

 doubted that the streams would be turbid with argillaceous mat- 

 ter, and, as well known, this would be immediately precipitated 

 upon coming in contact with salt water. Thus the material for 



