From none of the vents upon its slopes have all three kinds of lava escaped, but from the summits of Shasta and Shastina, which are the products of the two largest and most prolific vents, both hornblende and hypersthene andesite have been effused. All the other orifices were subordinate, and each furnished but one kind of lava; from seven of them came hypersthene andesite; eight, hornblende andesite; and the remaining five, basalt. The relative age of the cones which mark the position of the volcanic vents is indicated by the amount of degradation which each has suffered. Judged by this criterion, those of hornblende andesite are the oldest and those of basalt the youngest. The latter are for the most part made of lapilli, and are not crater-shaped as is usually the case in other portions of the Cascade Range, but are elliptical in form, with dome-shaped summits. The presence of considerable piles of ejectments about the subsidiary vents indicates that the eruptions from these orifices were often of a violent character. On the other hand there are some without a trace of lapilli, or anything else to indicate an interruption in the quiet flow of lava welling out of the depths.
Upon the eastern slope of the mountain the cañon, excavated by Mud creek, brings to light the oldest Shasta lavas now exposed, and they are seen under such circumstances that their succession can be readily understood. The oldest lava known is hornblende andesite, which is now in an advanced state of disintegration, and it seems probable that in the early stages of its development a large proportion of the lavas ejected from Mt. Shasta were of the same mineralogical constitution. These were succeeded by extensive effusions of hypersthene andesite. Later in its history, several small streams of hornblende andesite again burst forth from the northeastern side of the cone, but the final effort of the volcanic energy was spent in the ejection of hypersthene andesite. The conditions which determine the oscillation in mineralogical composition of the lavas are as yet conjectural, but when discovered, and their influence demonstrated, an important step forward will have been made in determining the relations of many volcanic rocks.
A striking feature in the structure of Mt. Shasta is the paucity of volcanic ashes, lapilli, and other ejected matter. Only one important deposit of the kind has been discovered. It clings about the summit of the mountain, and is evidently the product of its last eruption. The summit of Shastina is so regular in outline, and the shape of its crater so well preserved, that many have supposed it to be composed chiefly of scoria and ashes; but this is not the case, for its slopes are of angular fragments of compact lava.
Mt. Shasta is almost a pure lava cone, and its remarkably regular form is a matter of wonder. That it is so regular is a sequence of several favorable circumstances. Although a score of parasitic cones spring from the side of the mountain, and have contributed to its upbuilding, yet their additions have been so small compared with the vast effusions from the summit craters Shasta and Shastina, as not to greatly modify the outline of the mountain. More important circumstances are to be found in the non-explosive character of the eruptions and the successive changes in the physical properties of the erupted lava, as the development of the mountain progressed.
It is well known that among the volcanoes of the Hawaiian Islands the eruptions are quiet and effusive. The fiery streams of liquid lava course down the gentle slopes for many miles.
Although the mountain is 14,000 feet high, its lavas have such a high degree of liquidity, and retain their mobility so long after eruption, that the base of the mountain spread by them has a diameter of about seventy miles, and an average slope of 5° 1,800 feet below its summit. Mauna Loa is nearly twenty miles in diameter. On the contrary, at a corresponding position its greatest diameter is less than two miles, a very remarkable difference, which is due chiefly to the unequal fluency of the two lavas. The very oldest lavas of Mt. Shasta lie buried within its mass, and we know nothing of their physical properties, but from an examination of the oldest ones now visible, it is evident that at the time of their eruption they possessed a higher degree of fluidity, and were more voluminous than those of later date. The long, gentle slopes about the base of the mountain are formed by comparatively old lavas. Ascending the mountain, one goes up as if upon a giant staircase, with long, inclined steps rising abruptly over the ends of successive shorter and newer lava flows.
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.