Geology of Massachusetts. 7 
ded by the agitation of the waters. Some varieties of this diluvium 
may, indeed, be converted into a soil of tolerable richness by manur- 
ing it abundantly, and clearing away the stones. And generally too, 
the rains that have fallen on it for thousands of years, have conveyed 
its finer particles to the bottom of the vallies and cavities, with which 
this formation abounds, and these being mixed with much vegetable 
decayed matter, a soil of good quality is formed. So that within the 
limits of this formation, much good land occurs. But these fertile 
spots ought perhaps rather to be denominated alluvion than diluvium. 
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- 
lies, 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, and 
on the island of Nantucket, 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 eastern continent, by burying 
even villages and cities. The most effectual remedy that nature has 
provided against these encroachments, seems to be Beach Grass ;— 
(Arundo arenaria, Lin. Psamma arenaria, 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 Hoo- 
sac Mountain. None of them however are noticed on the Map. 
Most of the islands in Boston Harbor, are thus colored; also Plum 
island in Essex county, and a part of Malden 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 ; but 
in the tertiary formations, these materials are arranged in regular, 
and generally, in horizontal layers, one above another. 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 sur- 
