GLACIO-AQUEOUS ACTION IN NORTH AMERICA. 171 



2. Smoothed, Polished, and Striated Rocks. 



The facts under this head form a most important element in 

 the history of American drift. And yet they are more easily 

 overlooked than those respecting bowlders : nor can they be fully 

 appreciated but by long continued and careful observation. It 

 requires a good knowledge of the stratification, lamination, jointed 

 structure, segregated ridges, and peculiarities of disintegration in 

 different rocks, not to mistake these phenomena for striae and 

 furrows. Then it must be remembered, that where rocks have 

 been for ages exposed to atmospheric agency, most of them have 

 suffered so much disintegration, as to obliterate all superficial 

 smoothing and furrowing; or they are so coated with lichens as 

 to conceal such effects from superficial observation. But if any 

 one will always have an eye open for noticing the surfaces of 

 rocks, he will find many places, where neither the atmosphere 

 nor vegetation have been able to obliterate the traces of some 

 powerful agency. Coarse granite, gneiss, mica slate, and granular 

 limestone, thus exposed, will, indeed, rarely retain them ; but the 

 finer slates, whether micaceous, talcose, or argillaceous ; some of 

 the harder compact limestones, the fine grained syenites and traps, 

 some sandstones, and most conglomerates, seem almost to bid 

 defiance to the disintegrating agency of the atmosphere, and retain 

 these markings as fresh almost as if made yesterday. This is 

 especially the case ^vhen they have been protected by a fe^v feet 

 of soil ; so that the geologist will find the best examples where 

 roads, rail-roads, canals, and other excavations, have laid bare 

 new surfaces. In some cases, which have been particularly 

 described by Professors Dewey, Emmons, and Locke, in New 

 York and Ohio, the surface of hard limestone, over large areas, 

 has a polish almost equal to that of marble. The syenite and 

 porphyry of Essex county, in Massachusetts, exhibits a polish 

 almost equally good. Where the excavations for the Eastern rail- 

 road have newly uncovered these rocks, about three miles south 

 of Newburyport, also a little north of Ipswich, and between 

 Salem and Lynn, good examples may be seen on the east side 



