Observations on the Primitive Boulders of Ohio. 301 
by a stiff blue clay and‘is then called hard pan, but occasionally by 
carbonate of lime into a kind of pudding-stone. This is then di- 
luvial* earth and agreeably to the opinion of most geologists has its 
origin in some flood or deluge. 
If we travel hence, in a northern direction, this kind of earth 
becomes more and more common, till on the Sandusky plains it 
is the subsoil of the whole country. It sometimes covers the under- 
lying rocks to the depth of eighty or a hundred feet, as may be 
seen at the Bluffs near Circleville. At this place the Ohio canal 
is constructed along the base of a steep diluvial bank in such a man- 
ner that the strata are beautifully exposed for a distance of two 
miles. The clay found in this bluff is of two kinds—one a deep 
blue containing many fragments of a dark colored argillite with 
other kinds of gravel. It has probably resulted from the destruc- 
tion of this “kind of rock. The other is a yellowish kind of clay 
which covers the strata below in a non-conformable position. It 
contains fragments of the common limestone and sandstone of Ohio 
which are not much worn. ~ 
» The attentive observer will discover also, in travelling to the north 
that the gravel forms a larger proportion in the composition of the dilu~ 
vion and that the fragments are larger. In general when they exceed 
the size of one’s head they are called boulders by geologists ; but there 
is no distinct mark by which they are separated—they gradually 
pass into each other, and it is not always easy to determine wheth- 
er a particular stone should be called a boulder or a gravel stone. 
Proceeding still further north the boulders become more numerous ; 
they are of larger dimensions and generally less rounded by attri- 
tion. 
From all these facts it is evident that we must look beyond se: 
Great Lakes for the origin of our primitive fragments, and it only 
remains, to. point out the precise locality from which they have been 
removed and the question would be settled. We should then know 
every thing concerning them. But as our observations have not 
been extended into that region and as we have no books describing 
the rocks there, we must leave this to future observers. 
The surface of this immense deposit of diluvion every where 
presents a wave-like undulated appearance. These swells are gen- 
* Or perhaps tertiary as well as diluvial —E£d. 
Vou. XXII.—No 2. 39 
