426 
GEOLOGY OF THE FOURTH DISTRICT. 
and at Mendon and Pittsford. Beds of clay occur near Clarkson and Broekport; from some 
of which, bricks have been made. We might enumerate every town in the county, but the 
materials are similar, and are found in all. 
The decomposing gypseous marls appear at the surface in Pittsford, and other places, where 
they have been mistaken for clay. This marl contains too much carbonate of lime to be use¬ 
ful for bricks, but would be valuable if used as a manure on sandy lands. It makes bricks 
of a very inferior quality, which crumble on exposure to rains, and are peculiarly unfit for 
exposure to water. 
Sands. 
The only pure siliceous sands we find in this county are on the lake shore. With these, 
garnet and iron sands occur in small quantities. Coarse sand or fine gravel, fit for making 
mortar, is found in many places, particularly along the Irondequoit, and in many of the drift 
hills. The fine sand of these hills contains too much argillaceous matter to be useful where 
a siliceous sand is required. At the outlet of the Irondequoit bay, great quantities of a 
pure siliceous sand is drifted into ridges along the shore. Sand may be obtained from ibis 
place, in sufficient quantities for the manufacture of glass, or for sawing marble, should it ever 
be required for such purposes. This kind of sand is found at intervals, and in great quantities 
along the lake shore from the Irondequoit to Sandy creek. Magnetic iron sand and garnet 
sand are also found in considerable quantities, and collected for writing sand. The iron is not 
in sufficient abundance to be of importance as an iron ore. These two sands result from the 
destruction of granitic rocks containing both iron and garnet. 
Water and Springs. 
All the water of the county, and indeed of the whole district, contains lime in some form, 
being what is termed hard water. The county generally is well watered, though in some parts 
water is not so readily obtained. Along the northern slope of the “ Mountain ridge,” and 
over the whole of the sandstone formation, water is abundant, either rising to the surface in 
springs, or collecting in the low grounds and forming small streams. In digging wells, where 
the sandstone approaches the surface, it is often necessary to penetrate the rock a few feet 
before a constant supply of water is found. After ascending to the top of the limestone, wa¬ 
ter is not so abundant. Few springs rise to the surface, and the water accumulating in the 
low grounds and forming small streams, is dried up in summer, or lost in the fissures of 
the rock; consequently, at such seasons the supply of water on the surface is very limited. 
Water is obtained by digging, at different depths ; sometimes at the surface of the limestone, 
but commonly by penetrating it a few feet, and often to the depth of thirty or forty feet. In 
these cases it is usual to find a cavity or fissure in the rock, which affords an abundant supply. 
The water from the wells never overflows the surface, though from the dip of the rocks it 
might probably be made to do so. The water all flows into wells from the north, as we would 
