are overlain by a series of shales, sandstones, and limestones, whose structure in the area north of Arroyo Seco is shown by the beds of massive buff-colored sandstone that dip northeastward at an angle of about 45°. A western limb of this structure has not been recorded but may exist in the mountains further towards the coast. The observed dips at least suggest that Tassajara Hot Springs issue at a locality where Arroyo Seco crosses a zone of intense pressure in the underlying crystalline rocks. Analyses of two of the thermal waters showed them to be noticeably sulphuretted, and only moderately mineralized. The water of the arsenic spring has a distinctly yellow color, which in a few other springs has been ascribed to alkaline sulphides in solution. At the north edge of the creek, a few yards above the hot springs, there are two cool springs in which iron is deposited. An analysis of the easternmost of these springs was also made. The two hot springs apparently issue from the same general source and show only slight differences in composition. Primary salinity and primary alkalinity are the chief stable properties, but the waters are characterized by high subalkalinity, of which silica is the main component. The carbonate radicle reported is presumably calculated from the alkalinity determination and doubtless includes sulphides and possibly silicates. The apparent absence of arsenic in the so-called arsenic spring is noteworthy. Of markedly different character from the hot springs, the cool iron spring is less than half as concentrated and has secondary alkalinity as its dominant property. Subalkalinity is not reported, but is probably relatively low. The spring is probably of essentially surface origin, and not directly related to the thermal waters. Algous growths in the creek below the hot springs are recorded, as the growths are related to the sulphuretted character of the water. Although they are common to thermal sulphur springs, the relatively large volume of water in the creek at Tassajara Hot Springs, its comparatively slow cooling, and the presence of both swift currents and of quiet pools, affords an unusually good opportunity to observe growth variations. Water from two of the largest springs is piped to two plunge baths, and a vapor bath constructed over the hottest spring in the creek bed. History Esselen The springs were first used by native Esselen people who occupied the area for at least a thousand