3G0 
GEOLOGY OF THE FOURTH DISTRiCT. 
the debris of the strata it passes through, as well as much that has been brought in by other 
sources. The consequence has been a gradual filling up of the lake, until vegetation has 
taken root upon the bottom, and produced by its decomposition the extensive deposit of muck 
which covers the whole. There are similar deposits at the head of all these lakes, and in 
some cases considerable tracts have been reclaimed. 
Lake Marl and Tufa. 
In nearly all situations the muck swamps are underlaid by a deposit of calcareous marl. 
This is usually very finely pulverulent, and, though cohering when wet, is very friable when 
dry. When this calcareous deposit is made upon the surface, or in situations exposed to the 
air, it becomes a tufa or travertine, often preserving, in a most beautiful manner, the impres¬ 
sions of twigs and leaves, etc., so perfect that the species may be determined. 
This marl is derived from two sources, one being the limestone rocks themselves, and the 
other the calcareous particles distributed through the superficial detritus, the origin of which 
is still the limestone formations. The drift materials being composed, in a large proportion, 
of the debris of the rocks of the district, calcareous matter is widely diffused. This is not 
only a constant fertilizing agent in the soil, but, from the action of rains upon the surface, and 
the passage of the water through these superficial deposits, the calcareous matter is dissolved 
and carried forward into some lower situations, where it accumulates in the bottom of the 
small lakes and marshes. By this process, extensive beds of marl are formed in portions of 
the district remote from any limestone formation of the older rocks. These deposits occur in 
the southern counties, wherever the drift from the northern part of the district has accumulated. 
In situations above the reach of this influence, the soil has too little calcareous matter to pro¬ 
duce any such deposits, and consequently the elevated swamps in the southern counties are 
mostly destitute of it. This, however, is not universally the case ; for the bed of Casadaga 
lake, and the marshes around it, contain deposits of marl. 
Some of the most extensive formations of this kind in the district are made upon the Onon¬ 
daga salt group, and are deposited from the copious springs which rise along its southern 
margin. By referring to the head of Springs, under Corniferous limestone, it will be seen 
that there is a large surface drained through the fissures of that rock, and that this water flows 
out in the form of springs, on the north side of the terrace formed by it. 
These deposits of marl usually rest upon a bed of clay or sand, and are succeeded by muck. 
In the greater number of localities its formation has long since come to an end, but in others 
it is still in progress. In many of the springs issuing from the rocks, its daily deposition can 
be observed; it incrusts all the vegetables growing in the stream, and, in favorable situations, 
forms deposits of considerable extent. The tufa is used for building stone ; being soft, when 
first removed from its bed, it is easily cut, or hewn into blocks of convenient size. These, 
after drying, become comparatively hard, and form a durable material. There are, however, 
but few situations where there is a quantity sufficient to allow of its being used in this manner. 
