Tertiary Formations. 7 



Had diluvium been represented on the Map wherever it occurs, 

 scarcely any other formation could have been exhibited. I have 

 marked the region as diluvial, only where it occurs in such quanti- 

 ties, as almost entirely to conceal every other stratum. It is most 

 abundant in the south east part of the State ; the counties of Plym- 

 outh, Barnstable, Dukes, and Nantucket, being almost entirely over- 

 spread by it ; so that in the three latter counties, I scarcely found any 

 rocks that did not appear to have been broken up and moved from 

 their original bed. Towards the extremity of Cape Cod, this stratum 

 is composed almost entirely of sand ; which often constitutes those 

 hills called downs or dunes* that travel inland by the action of winds, 

 and do great mischief, by overrunning fertile spots ; and on the east- 

 ern continent, by burying even villages and cities. The most effec- 

 tual remedy that nature has provided against these encroachments, 

 seems to be Beach Grass ; (Arundo arenaria, Lin. Psamma are- 

 naria, Beauv.) which is able, not only to fix itself on the most barren 

 ridge of sand, but also in time to fix the sand itself. 



Diluvial tracts of considerable extent exist in the county of Nor- 

 folk, in the Connecticut valley, and along the western base of Hoosac 

 Mountain. None of them however are noticed on the Map. Most 

 of the islands in Boston Harbor are thus colored ; and a part of Mai- 

 den and Chelsea. 



Tertiary Formations. 



The only difference between these and diluvium, is, that in diluvi- 

 um, the sand, pebbles, and clay, are confusedly mixed together ; ex- 

 hibiting only an imperfect stratification ; but in the tertiary formations, 

 these materials are arranged in regular, and generally in horizontal 

 layers, one above another, and appear to have been deposited at 

 earlier epochs. Hence, When the sandy stratum happens to lie 

 uppermost, the soil will be too sandy ; but if this be worn away, 

 so that the clay lies at the surface, the soil will be too argillaceous ; 

 or if the gravel stratum be exposed, the soil cannot be distinguished 

 from diluvium. Most of the varieties of soil thus produced may be 

 seen in the valley of the Connecticut ; where exists the most extensive 

 tertiary formation in the State ; extending nearly to Middletown in 

 Connecticut. Upon the whole, there is little to choose in an agricul- 



* Dunes are properly alluvial phenomena, though diluvial sand (as in the case 

 mentioned in the text) is often employed in their production. 



