260 
GEOLOGY OF THE FOURTH DISTRICT. 
new and interesting features. The differences in the character, both of strata and of fossils, 
on which subdivisions may be founded, will appear after an examination of these localities. 
Thickness. — This group, in the eastern part of the district, can scarcely be less than 1500 
feet thick. This thickness is estimated from the dip over a distance of thirty or forty miles, 
and the height of the hills towards the southern boundary of the State, which rise to eight 
hundred feet above the valleys. 
The summit of the Portage group, on the Genesee river, is less than 1200 feet above tide 
water, and the lowest passes in the hills south of this, occupied by the Chemung group, are 
from 1500 to 2000 feet above tide water. The highest hills toward the south part of the 
State are scarcely less than 2500 feet above tide water, showing a difference of elevation 
between the two groups of 1300 feet. Allowing for undulations, which render the dip irregu¬ 
lar, the whole thickness is above 1500 feet. At the western limit of the State the group has 
evidently thinned to a considerable degree, though no good opportunities of measuring were 
presented. When examined beyond the limits of the State, the evidence of its diminution 
becomes more apparent; and when we go as far as Indiana, the whole of this group and the 
Portage is embraced in a thickness of less than 400 feet. Still farther west, it is very pro¬ 
bable that they have disappeared altogether. 
Mineral contents of the group. — There is little of interest throughout the whole extent of 
this group. Some of the septaria present the same minerals as those in groups below. Iron 
pyrites often takes the place of fossils, and carbonate of iron is very commonly found replacing 
the stems of crinoidea and some other fossils. This mineral becomes more abundant in the 
upper part of the group, and is rarely seen in the lower part. The decomposition of the rocks 
gives rise to the sulphates of iron and alumina, showing the presence of iron pyrites very 
universally diffused. The strata are often stained by the oxide of manganese, and the pro¬ 
duction of wadd in many places proves its occurrence in the strata in considerable proportion. 
Springs. — The same remarks apply to that portion of country occupied by this group of 
rocks as the last. When covered by forests, the surface is well watered by perennial springs 
and streams in great numbers. As the improvements progress, the wood is gradually cut from 
the higher grounds and the broad sloping hill-sides; the surface being thus laid open to the 
direct rays of the sun, many of the springs fail, and the small streams are dried up. The 
present system of clearing the country, and the wanton destruction of the forests, will even¬ 
tually produce serious evils, in the want of water; and the inhabitants should remember, that 
unless they obey the laws of nature in this respect, their sins will be visited upon their chil¬ 
dren, and they will be driven out, and a stranger shall possess the land. This subject will be 
farther noticed under the chapter on agriculture. 
Agricultural characters. — The soil resulting from the decomposition of the rocks of this 
group is a compact clayey loam, which, with the great abundance of angular fragments of the 
rock, gives it the character which is termed “flat gravel .” The soil of the valleys, particu- 
