GEOLOGICAL SCALE AND STRUCTURE. 5 



as West Canada Creek, makes a rapid descent in this township, from the 

 Adirondack uplands to the Mohawk Valley, falling three hundred feet in 

 two miles, by a series of cascades. These cascades have long been 

 known as the Trenton Falls, and the limestone which forms them was 

 appropriately named, by the New York geologists, the'Trenton limestone. 

 The formation, as seen at the original locality, is a dark blue, almost 

 black, limestone, lying in massive and even beds,' which are sometimes 

 separated by thin layers of black shale. But it is to be noted that a few 

 feet of its uppermost beds consist of crinoidal limestone of great 

 purity of composition. Both limestone and shale contain excellently 

 preserved fossils of I,ower Silurian age. By means of these fossils, and 

 also by its stratigraphical order, the limestone is , followed with perlect 

 distinctness from Trenton Falls to every point in the. compass. It is 

 changed to some extent in color and composition, as it is followed in 

 different directions, but there is seldom a question possible as to its iden- 

 tity. The Trenton limestone forms several of the largest islands, in 

 whole or in part, in the northern portion of Lake Huron, as the Mani- 

 toulin islands. From this region it dips, under cover of the lake and 

 also of higher formations, to the southward; but it is found rising again 

 in outcrop in the valley of the Kentucky River, near Frankfort, and pos- 

 sibly, also, at a single point within the limits of the state ot Ohio, viz.: 

 in the quarries of Point Pleasant, which are located in the valley of the 

 Ohio River, in Clermont county, twenty miles above Cincinnati. The 

 Point Pleasant beds have a thickness of about fifty feet. This outcrop 

 of rock was definitely referred to the Trenton horizon, apparently on 

 stratigraphical grounds, by the late W. M. L,inuey, of the Kentucky Ge- 

 ological Survey, a number of years ago ; and in the course of the explo- 

 rations of our underground geological structure, which followed the 

 search for oil and gas in western Ohio, the indications seemed to point to 

 the correctness ot Mr. Linney's conclusion, and the Point Pleasant beds 

 have been counted Trenton limestone in several of the last volumes of 

 the Geological Survey. This determination has, however, lately been 

 called in question by Mr. Joseph F. James, of the U. S. Geological Sur r 

 vey, who bases his argument ou.paleontological grounds. Mr. James 

 holds that it is impossible to separate the Point Pleasant beds from the 

 overlying series, which are referred to the Hudson River age. It is not 

 safe to affirm positively that the determination made by Mr. Linney, and 

 supported by the facts brought to light in the Ohio reports, is beyond 

 question ; but it still seems probable that this is the true interpretation. 

 Th ; fossils of the Trenton limestone and of the Hudson River group are 

 identical to a considerable extent. 



The thickness of the Trenton limestone proper, as it appears in out- 

 crop in the rocks of central Kentucky, is given by the geologists of that 

 state as one hundred and seventy-five feet. It is immediately underlaid, 

 in this region, by two other limestones, viz.: the Birdseye and Chazy, 



