It is evident in comparing the older and newer lava flows of Mt. Shasta that there has been a more or less regular decrease in the quantity of lava extruded during successive eruptions, and this is exactly what we should expect when we consider that as the pipe is lengthened by successive effusions, the hydrostatic pressure of the columns of lava within is gradually augmented. The increased compress of the lava flows toward the summit of the mountain indicates that the lava of successive extrusions became more and more viscous until at last the eruptions became explosive, and gave rise to the ejectments now clinging upon the upper slopes of the mountain to evidence the character of the final outburst.
It is not only possible, but very probable that the increased viscosity of lava toward the closing scenes of the volcano is correllated to the diminution of temperature. Since the beginning of the historic period there have been no eruptions from Mt. Shasta, but the freshness of its lavas indicate that not many centuries ago, with other volcanoes of the Cascade Range, it was in a state of vigorous activity, and groups of hot springs and fumeroles about the summit still attest the presence of smouldering volcanic energy, which may perhaps some day break through its confining walls.
The upbuilding of Mt. Shasta is but a matter of yesterday, as compared with the lapse of ages, since the birth of some of its neighbors. The complex group of mountains to the westward, embracing the Scott, Trinity, Salmon and Siskiyou, are composed in large part, at least, of ancient crystalline rocks of both aqueous and igneous origin; through these the rivers have cut deep cañons, the Klamath, on its way to the Sacramento southward, from the very base of Mt. Shasta to its broad valley stretching from the Sierra Nevada to the Coast Range. The cañon of the Sacramento was cut down to nearly its present level, and the mountains sculptured into existing forms long before the eruptions of Mt. Shasta had ceased, for a fiery deluge escaping from the southern slope of Mt. Shasta entered the Sacramento cañon, and as a lava stream 200 feet deep followed its course for over fifty miles.
Towering more than a mile above its neighbors, perhaps the youngest of the group, Mt. Shasta is the end of a long series of volcanoes in the Cascade Range, stretching northwest to Mt. Tacoma. This range, composed chiefly of volcanic material, is cut across by the cañons of the Columbia and the Klamath rivers, in the former of which, beneath a thickness of 3,500 feet of lava, are found strata containing Tertiary fossils. At the southern base of Mt. Shasta, in the cañon of the McLoud River, similar beds of volcanic debris are found, but without fossils, nevertheless it is evident that the main mass of the Cascade Range and its volcanoes originated in recent geologic times, and from the fact that solfataras, fumeroles, and hot springs are still abundant upon their slopes, they can not be reckoned among those which are wholly extinct.