The rocks of the Hamilton group crop out in a narrow belt in New York and Pennsylvania, which forms a great semi-ellipse extending from Lake Erie in the vicinity of Buffalo eastward to the Hudson River valley near Albany, thence south and southwest into Pennsylvania and New Jersey near Port Jervis, New York, and thence by the curving highly folded Appalachian belt westward and southwestward through Pennsylvania into Maryland. The area within this semi-ellipse is approximately 250 miles east and west and 125 miles north and south.

The Hamilton is buried under younger Devonian rocks within this area and practically no information was available concerning its character until the discovery of natural gas in the Oriskany sand in Schuyler County, New York, and Tioga County, Pennsylvania, in 1930. Since then more than 100 scattered deep wells in southern New York and northern Pennsylvania have been drilled through the rocks composing this group from most of which samples of drill cuttings have been collected and described.

Studies of logs of these wells have revealed certain information about the Hamilton which will be of interest to those engaged in solving the complex problem of the lateral change in the Devonian strata in this region and of usefulness to those geologists engaged in the oil and gas industry.