96 
GEOLOGY OF THE FOURTH DISTRICT. 
Mr. Vanuxem suggests that this structure is due to the crystallization of sulphate of mag¬ 
nesia in the fibrous form, which penetrated the rock, and produced the striated surfaces ; that 
it has since been dissolved, and that probably the same water which dissolved the saline matter 
deposited the carbon which invests the surfaces. We find in many instances where this struc¬ 
ture occurs, that the interstices are still occupied by carbonate of lime in long fibrous crystals ; 
and it is suggested whether this mineral, as well as some others, may not have been the cause 
of their production. These appearances are very obvious in many places where there has 
been a fresh exposure, and weathering removes the carbonaceous matter.* 
Topographical features. — In the eastern part of the district the limestone is thin, offering 
little protection to the shale, which is consequentlj'- worn down to a very gentle inclination, 
presenting a much greater breadth of surface than farther west. The limestone, becoming 
thicker in the same direction, occupies a broader space, while the shale is seen only in its 
outcrop. The nature of the two masses has given origin to much interesting scenery along 
its northern margin; every stream crossing it has its rapids or cascades. The fall and deep 
gorge at Wolcott village is the first of importance within the district. Several points of minor 
interest occur between this and the Genesee river; but from the great accumulation of drift, 
and the absence of large streams, the rocks are not well exposed. On the Genesee, the group 
gives origin to the upper fall at Rochester, the rapids above, and the cliffs below, which to¬ 
gether display both divisions in great perfection. The falls at Shelby, three miles south of 
Medina, on the Oak-orchard creek, are over the lower limestone and shale of this group. The 
“ mountain ridge,” before alluded to, continues a distinctive feature of the scenery from the 
Genesee to the Niagara river; and the numerous deep and picturesque gorges in the margin 
of this cliff sufficiently indicate the former existence of streams of great magnitude, or an 
excavating power operating from the north. 
Finally, this group gives rise to the grand Falls of Niagara, the wonder of the world, and 
a geological monument by which to measure its periods, and enable man to compare the past, 
the present and the future, and thence deduce the formula for determining the duration of 
those eras in years, which are now only known by the exhibition of phenomena which cannot 
indicate dates, or computation of time in human periods. 
Localities. — The principal localities have been enumerated in the description of the group ; 
a few of these will give the observer an acquaintance with the variations in character which 
the rock assumes in its western extension. The most eastern localities are in the towns of 
Wolcott and Butler, the former exhibiting the shale and the latter the limestone. In order to 
observe the gradual changes which the limestone undergoes before its full development at 
Lockport and Niagara Falls, a few other localities in Wayne county should be examined, as 
the quarries in the towns of Williamson and Marion. Along the Genesee at Rochester we 
find one of the best exposures in the whole district, and this is a very desirable point for in- 
See also the same structure represented under the Onondaga-salt gtoup. 
