926 
187. 263 cnéslewt 
This last and lowest bed visible at the north end of the exposure is cal- 
culated by Mr. Sanders to lie 1280 feet above the lowest bed of the 
whole series of Cambro-silurian limestones, or in other words, 1280’ higher 
in the Paleozoic system than the Potsdam Sandstone. This calculation is 
made by projecting curves according to all the observed dips between the 
north end of exposure, and the edge of the slate country to the north, 
checked by dips in a series of exposures on the east bank of the river. It 
is not necessary to go here into a discussion of the existence of a great up- 
throw fault between the limestones and slates. 
The topmost bed, No, 1, of the series is therefore 1280’ + 426’ = 1706’ 
above the Potsdam Sandstone. 
The upper limit of the limestones, where they pass conformably beneath 
the Utica or Hudson River slates, is seen several miles down the river. By 
projecting dip-curves in the interval Mr. Sanders measures an additional 
thickness of limestones amounting to 1819 feet. 
The total observed thickness of the great limestone formation (No. II of 
the old Pennsylvania Survey including representatives of Calciferous, 
Chazy, Birds’ eye, Black-river, and Trenton limestones) is 3535/. 
The beds selected for examination lie therefore a little below the middle 
horizon of the mass, and undoubtedly belong to the ‘‘Calciferous Lime- 
stone Formation’’ of the New York geologists, the Magnesian Limestone 
Formation of the Western geologists. 
All these beds contain carbonate of macnesia ; but while in some of 
them the percentage of this element is very low, as low as 1% or 2%, it 
rises in others very high, even to 37%. Some of the beds may therefore be 
spoken of as pure limestones, and others as true dolomites. The remarkable 
features however are: 1. That by virtue of some unknown law very few 
of the beds seem to occupy an intermediate place or exhibit a mixed or 
moderate character; and 2. That the two extreme types alternate, every 
other bed being limestone, and every other bed being dolomite. 
The astonishing regularity of this law is not so evident to one who 
merely reads the table of analyses, but unmistakably forces itself upon the 
attention if the reader converts the table into a diagram. Whether the law 
rules over the sequence of the whole series of 98 beds is yet to be discov- 
ered. Evenif it does we cannot safely formulate it as a law determining 
the distribution of the carbonate of magnesia throughout the 3535 feet of 
Siluro-cambrian limestones. But such a law, whatever be its restrictions, 
demands the earnest attention of chemists and geologists. 
The following table then gives the percentage of carbonate of magnesia 
in each bed: in column A at railway grade; in column B at the top of 
the cut, or quarry. Also, the carbonate of lime, column C, railway grade, 
column D, top of cut. Also, insoluble material, column E, at grade, column 
F, at top. 
