1875.] 417 [Dodge. 



common among and apparently derived from the amygdaloids and 

 slates, sometimes hold small quantities of specular oxyd of iron and 

 usually contain calcite and anhydrite. Galena occurs occasionally. 

 The body of the amygdaloids, as well as the nodules and veins, 

 sometimes contains small quantities of red oxyd of copper and mala- 

 chite, very likely the result of the decomposition of copper pyrites. 



Glacial Effects. 



Between the formation of the solid rocks described, and of the loose 

 soils which rest upon these and are for the most part derived from 

 them, is a long interval. There are some interesting points to be 

 noticed in the action upon the peculiarly situated rocks of this vicin- 

 ity of the great ice-sheet, now generally believed to have covered the 

 northern part of this continent in geologically recent times. 



In the great natural glacis of crystallines which surrounds the 

 Boston basin there is a large gap now occupied by the line of ponds 

 in Woburn, Winchester and Arlington. This valley across the 

 ridges of the rock was probably formed, to a considerable extent, by 

 glaciation, the rock being at that point less durable ; for where great 

 eruptive masses occur, they still stand prominent as hills in the midst 

 of the depression. The course of the ice seems to have been consid- 

 erably guided by the conformation of the surface over which it 

 passed. Here the direction is several degrees to the east of south. 

 In Billerica, some deep furrows on granite and mica schist indicate 

 about S. 25° E.; in Woburn, S. 20° E., about the course of the 

 chain of ponds in that valley. 



In the basin of stratified rocks, the ice, following this course of 

 about S. S. E., generally strikes the upturned strata almost at right 

 angles to their strike. In Somerville, the slates lying N. W. - S. E. 

 are polished on their stratification planes in some cases where the 

 ice masses slid over the sloping sides of hills running in the direction 

 of their course, as at the Alms House Quarry, Cambridge. In all 

 cases the erosion has been great. Often the basset edges of hard 

 slate strata are planed off as level as a floor. A good illustration 

 of this action on a horizontal plane is at the quarry near New- 

 ton Corner, lying partly in Watertown, in " Morse's field." 



But irresistible as the power at work seems to have been when it 

 met with equal opposition at all points, its plasticity is even a more 

 noticeable feature. Thus, while the average of fifty-three observa- 



PROCEEDINGS B. S. N. H. — VOL. XVII. 27 JULY, 1875. 



