14 
GEOLOGY OF THE FOURTH DISTRICT. 
continued they would meet in the same plane ; and pursuing the same course in regard to 
near or distant hills, or across broad valleys, we find invariably the same rule to hold good. 
These features are produced only by the removal of the mass which once filled the space, 
precisely in the same manner as those made by the excavation of roads through ridges of sand 
and gravel; the power in this case being that of nature over a great extent of surface, and 
in the other that of man over a very small one. All these hills are termed in geology outliers , 
or continuations of the same strata, where some intervening portions have been removed. 
The larger streams flow in the deepest valleys, while the intermediate portion of country 
is less excavated, and presents eminences less abrupt. Much of the higher ground, indeed, 
exhibits a surface with gentle eminences and broad valleys, bounded by low hills equally 
extensive. This character is more extreme toward the southern limit of the State ; and on 
going northward, gradually diminishes, the undulations becoming more gentle, until finally we 
come upon an almost unbroken level. 
Further examination will show that each change in the topographical features is due to a 
change in the underlying rocks, and that the same rock does not give rise to any two of the 
main features described. The place and limit of certain formations are thus indicated upon 
the surface, and afford a general guide to the observer; though he will find less important 
changes in the strata not affecting the external form or character of the country. The thick¬ 
ness or extent of a rock, also, has great effect in modifying the external exhibition ; while 
thin masses of different kinds may exist interstratified with thicker ones, and the whole sur¬ 
face carry the character of the predominating rock. 
On investigating the lithological character of the strata of this district, we shall find that 
their varied composition and texture has given rise to the great features of its surface; and 
that hence is due not only those pleasing and beneficial inequalities, but also the origin of the 
streams and water falls which beautify and enliven the scenery, while they offer encourage¬ 
ment to enterprise and industry, which in a country less diversified would never be called 
into action. We find the first plateau, or that bordering Lake Ontario, underlaid by a soft 
friable or shaly sandstone, having nowhere, except in its higher members, sufficient firmness 
to resist the universal denuding action which has formerly prevailed far beyond the limits of 
this district. Accordingly it is evenly worn down, and presents few varied features, except in 
the river channels. Through Wayne and a part of Monroe counties, a portion of this gentle 
slope toward the lake is underlaid by some thin beds of limestone with alternating and suc¬ 
ceeding shales, giving no different aspect to the surface. Through this part of the country, 
also, the terrace bounding the plateau is scarcely defined, forming merely a slight elevation 
above the country on the north. Westward from Rochester, it becomes a prominent feature ; 
and the shales just alluded to form its abrupt northern slope, increasing in height as we go 
westward. The platform of this terrace is a thick mass of limestone, which has resisted the 
influences that levelled the shale and friable sandstone on the north ; and it now presents its 
line of strike in bold relief, extending from Rochester to the Niagara river at Lewiston, and 
far beyond into Canada. 
I 
