The earliest extensive studies of thermal springs in the United States were made by physicians. In 1831 Dr. John Bell issued a book entitled "Baths and Mineral Waters" in which he listed 21 spring localities. In the edition of his work published in 1855 the number was increased to 181. The earliest report on a geologic study of thermal springs was that of W. B, Rogers in 1840 on the thermal springs of Virginia. In 1875 G. K. Gilbert published a map and table showing thermal springs in the United States and pointed out that they are present chiefly in the mountainous areas of folded and faulted rocks. Early geologic study of them was principally inspired by the information which they afford at a few places on the deposition of minerals. The relation of hot springs to volcanic action has been studied in the Yellowstone National Park and near Lassen Peak in California. Studies in recent years have been concerned with the source of the water as well as of its heat.

All the notable thermal springs in the eastern United States are in the Appalachian Highlands, principally in the region of folded rocks. The Atlantic Coastal Plain contains no appreciably warm springs. In Florida there are large springs whose water rises from a depth of a few hundred feet and is about 5° above the mean annual temperature, but they are not usually classed as thermal.

The only warm springs in the great Interior Plains region are at and near Hot Springs, S. Dak., in the vicinity of the Black Hills uplift of crystalline rocks. In the Interior Highlands thermal springs occur only in the Ozark region, the largest group being at Hot Springs, Ark.

The Rocky Mountain System includes the Yellowstone National Park, with its world-famous hot springs and geysers (see pis. 7,12), and there are many other hot springs within this great mountainous region. In the Intermontane areas of great lava plains and faulted lava mountains in Utah, Nevada, southern Idaho, and eastern Oregon there are many hot springs, closely associated with the larger faults. In the Pacific Mountain System, including the Cascade Range and Sierra Nevada, there are many warm and hot springs, some of which issue in areas of granite, and others in areas of lava. In the Coast Ranges of California many thermal springs issue from different geologic formations.

Of the total of more than 1,000 thermal-spring localities listed in this paper more than half are situated in the three States of Idaho, California, and Nevada, each of which contains more than 150 thermal-spring localities. Wyoming, including the Yellowstone National Park, contains more than 100 hot-spring localities. Oregon, Utah, Colorado, Montana, and New Mexico contain several dozen thermal springs each, of which the principal ones are developed as resorts. The other thermal springs are scattered through 12 States, of which Massachusetts, New York, Pennsylvania, and North Carolina contain one spring or group each. More than half of the total number are developed as resorts or used for irrigation or water supply, but many have remained undeveloped because they are not easily accessible.