Devonian Era in the Ohio Basin. — Claypolc. 81 
reaches its maximum thickness at or near the Susquehanna 
gap through the Little mountain, four miles north of Harris- 
hurg, where its vertical beds are nearly 800 feet thick. From 
this point it thins off in all directions, splitting into a lower and 
upper division with intervening shale, as in Huntingdon county, 
and becoming itself more and more shaly until at a distance of 
about fifty miles from its center the whole sheet disappears and 
the Hamilton resumes its usual shaly composition*. 
It is a legitimate inference from the presence of this Ham- 
ilton sandstone that somewhere on the eastern land, at no great 
distance, a local elevation was taking place and that from it was 
swept off the coarse material by some river whose mouth lay 
probably east or southeast of the spot where now stands the 
•city of Harrisburg or where that spot lay before the region was 
subjected to the folding of the Appalachian revolution.! 
What changes, if any, in the sea-margin are involved in this 
elevation it is not possible to determine in consequence of these 
disturbances, whereby the strata have been folded into close 
anticlines and synclines whose beds are often vertical or even 
overthrown, in some places severely faulted and occasionally 
overthrust, so that some are completely concealed by others that 
have been forced over them along shear-planes developed by 
the enormous pressures to which they have been subjected. 
Consequent erosion has removed all that portion that once lay 
to the southeast of their last outcrop at the Susquehanna gap 
and has destroyed the materials out of which their history could 
have been written. 
Outside of these two areas no disturbance seems to have 
marked the Hamilton period. Its strata in western New York 
are partly calcareous, indicating a sea less clouded with shore 
wash and affording a typical Hamilton fauna. From this area 
which occupied the central portion of the gulf the sediment and 
the fauna gradually shade off until, as above mentioned, in the 
far west it is not possible to distinguish from one another the 
different periods of the Devonian. 
In New York, however, professor Hall was able to separai 
the Hamilton into three parts, both lithologically and palseon- 
tologically. They are as given below: 
* For further details of Hamilton, consult reports of 2nd Geol. Survey ol 
Penna. on the counties named, especially Perry (F2), E. W. Clavpoi f ami 
Huntingdon fJ3), I. C. Whith. 
t E. W. Ci.AYi'OLE. Am. Sat., March, 1885. 
