In contrast to the flow of surface springs, which varies with the season through wide ranges both in its volume and in temperature of the water, the volume of fissure springs is but slightly affected by the seasonal precipitation, and the water temperature is maintained relatively constant. Rock is but a poor heat conductor, and the seasonal temperature changes descend a few feet only into the ground. Thus water which rises from depths of a few hundred feet only is apt to be icy cold, while from greater depths the effect of the earth’s internal heat is apparent in a uniform but relatively higher temperature of the water. Such “warm” or _thermal_ springs are apt to contain considerable mineral matter in solution, both because the water is far traveled and because its higher temperature has considerably increased its solvent properties. It has long been recognized that lines of junction of different rock formations at the base of mountain ranges are localities favorable for the occurrence of thermal springs. These junction lines are usually within zones where by movement upon fractures the widest openings in the rock have formed, and the catchment area of the neighboring mountain highland has supplied head for the ground water. A map of the hot springs within the Great Basin of the western United States would present in the main a map of its principal faults. =Artesian wells.=—From the natural fissure spring an artesian well differs in the artificial character of the perforation of the impervious cover to the water layer. The water of artesian wells may flow out at the surface under pressure, or it may require pumping to raise it from some lower level. Ideal conditions are furnished where the geological structure of the district is that of a broad basin or syncline. The water which falls in a neighboring upland is here impounded between two parallel, saucer-like walls and will flow under its head if the upper wall be perforated at some low level (Fig. 201, 3). [Illustration: FIG. 201.—Schematic diagrams to illustrate the different types of artesian wells, (1) A non-flowing well; (2) flowing wells without basin structure caused by clogging of the pervious formation; (3) flowing wells in an artesian basin. The dotted lines are the water levels within the pervious layers (after Chamberlin).] A monoclinal structure may furnish artesian conditions when the generally pervious layer has become clogged at a low level so as to hold back the water (Fig. 248, 2). Pumping wells may be used successfully even when such clogging does not exist, for the slow-moving underground water flows readily in the direction of all free outlets (Fig. 201, 1).