1870.] 197 [Shaler. 



bridge and part of Medford, the same sort of ridges with a similar 

 trend. One of these ridges beginning in Charlestown, and continued 

 through Somerville and Medford to the borders of West Cambridge, 

 has a length of five miles. At certain points in this group of drift 

 hills we perceive indications of the northeast and southwest ridges, 

 but on the whole the ridges conform in course with those in Chelsea. 



On the south side of Charles River, some of the loftiest ridges, 

 such as Corey's Hill, are composed for the upper hundred feet, at least, 

 of this drift material, and the direction of their axes is approximately 

 the same as those in Cambridge. In Boston and South Boston the 

 drift hills, though not so clearly defined as those just mentioned, seem 

 to belong to the same class of northwest and southeast ridges. A more 

 extended comparison of the courses of the drift hills of the coast 

 would not be desirable here ; enough has been given to make it clear 

 to those who are conversant with the facts that there exists the same 

 system of trends in the ridges on the main land that Count Pourtales 

 has pointed out in the harbor. 



It is an interesting question to determine whether these ridges have 

 been formed as such, or are the remnants of more extensive masses; 

 upon the determination of this question must rest many important 

 conclusions concerning the nature of the operations which took place 

 during the glacial period. 



A comparison of the sections given at various points in the islands 

 of the harbor, at Chelsea, Somerville, Cambridge, Brighton, South 

 Boston and elsewhere, has shown that through this region the drift is 

 remarkably similar at the same height above the sea. The mass of this 

 drift has a structure which is of a very inexplicable character; it cannot 

 be called stratified in any ordinary sense of the word; the pebbles are 

 of various sizes, from five foot boulders down to coarse sand, but the 

 whole packed in a fine mud, which so binds the materials together that 

 in the lower parts of the mass, where it has been subjected to consid- 

 erable pressure, it is almost as hard as parts of the Roxbury conglom- 

 erate, and resembles it in a most surprising and suggestive manner. 

 This peculiar feature of a mud cement binding the mass together has 

 been long ago remarked by Professor Agassiz as unquestionable evi- 

 dence of the fact that this mass could not have been deposited under 

 the ordinary conditions of stratified materials. In sections such as are 

 exhibited on Somerville hill, or at several points in Chelsea, this mass 

 is seen to have a thickness of at least one hundred feet. Throughout 

 this extensive section there is nothing which can be recognized as 



