MARL AND TUFA. 
361 
In the bottoms of all the lakes in the northern and middle parts of the district, the Chara 
abounds, as it also does in the beds of streams, where the water is charged with calcareous 
matter. By dredging along the bottom, it may be brought up in large quantities, and the same 
takes place after high winds. The prevalence of a high south wind is sufficient to strew the 
shores of Seneca and Cayuga lakes with this vegetable. It is always incrusted with, indeed 
apparently almost changed to, carbonate of lime. It is at first green, but by exposure soon 
becomes white, and crumbles to powder in the hand. 
In the outlet of the Caledonia spring this vegetable grows so rapidly, and so abundantly, 
that it chokes up the mill-races, and requires to be constantly removed. The presence of 
carbonate of lime seems favorable to its growth ; for in other similar situations, where this is 
absent, it does not flourish. 
In the greater number of the marl beds the remains of fluviatile testacea are very abundant, 
though it is only in a few situations where they have formed any large proportion of the de¬ 
posit. The shells appear to have flourished in immense numbers, probably from the facility 
with which they obtained calcareous matter, and other favorable circumstances ; but still it is 
plain that the formations of this kind are generally due to calcareous springs, or to the perco¬ 
lation of rain water through the surrounding rocks, which, from its excess of carbonic acid, 
dissolves the calcareous particles in the soil or the harder strata. 
The shells occurring in these deposits are all of recent species, and indeed the same may 
often be found, still living, in great numbers, in the marshes above the marl. The Limnea 
jugularis, Planorbis bicarinata and two or three other species, Valvata tricarinata, and 
Cyclas similis, are among the most abundant species. 
Discoloration of sands and clays from the percolation of water. 
The discoloration of sand and clay beneath muck swamps, or vegetable deposits of any 
kind, is deserving of notice as sometimes leading to important conclusions. The green lines 
and patches in the Medina sandstone have been attributed to the deoxidation of the iron, from 
the presence of carbonaceous matter. The same is noticed in the Old and New red sand¬ 
stones, and indeed in all rocks colored by oxide of iron. In the superficial deposits, the pre¬ 
sence of free carbonic acid renders the water capable of dissolving and removing the coloring 
matter, forming deposits of the hydrous peroxide of iron. This takes place with great facility 
in muck swamps, or where there is a deposit of vegetable matter above the soil. In this way 
we are to account for the white gravel and sand found in swamps and bog meadows. In some 
cases the change is only partial, and in others there seems a change from a lower to a higher 
state of oxidation. 
We usually find two kinds of clay noticed ; the blue and yellow, the latter succeeding the 
former, and generally supposed to be a subsequent deposition. This may perhaps be true in 
many localities ; but in Western New-York, not only the clays, but the other superficial de¬ 
posits often take their color from the effect of the percolation of w r ater, which probably changes 
the state of oxidation in the coloring matter. The surface deposits have originally been of the 
[Geol. 4th Dist.] 46 
