734 DYNAMICAL GEOLOGY. 



through opened fissures, for the hydrochloric acid in the vapors and 

 the chlorids among the saline effloresences indicate a marine source 

 for part of the waters. 



Its eruptions have taken place at very irregular invervals, and with increasing fre- 

 quency in later times. At the commencement of the Christian era it was a vine-clad 

 mountain, apparently a feurnt-out volcano. In 79 A. D., after several years of occas- 

 ional earthquakes in the region, the fires broke out anew, amid heavy shakings. The 

 summit of the mountain was engulfed, leaving Somma to mark the former northern 

 limit. No lavas outflowed, as far as known, but cinder ejections occurred on a vast 

 scale, and the rain from the clouds of vapor, produced, with the fallen cinders, a flow- 

 ing mud which buried the cities of Pompeii and Herculaneunt. The partly hardened 

 mud is tufa. Many eruptions of Vesuvius have occurred since, but, until recently, only 

 at very long intervals. Nearly a thousand years passed before the next recorded erup- 

 tion, or that of 1036 ; and a century or more intervened between the most of the out- 

 breaks of the next 600 years. In 1631, both lavas and cinders proved destructive to 

 cities and villages around the mountain ; but, for years before, the crater, then five miles 

 in circumference, had been a grazing ground for cattle. Since 1631 the eruptions have 

 been very numerous. Vesuvius is hence an example of a volcano which has increased 

 in activity during the past 2,000 years. 



Besides ejections of fused material, the volcano has thrown out in former times masses 

 of limestone and other rocks, which were torn from non-volcanic terranes along the 

 throat of the volcano. From these masses come the vesuvianite, humite, mica, and 

 rarious other species in well-crystallized specimens which are obtained about Somma. 



4. Mount Shasta. — Mount Shasta, in Northern California, is one 

 of a series of great volcanic mountains on the Pacific border of North 

 America. It is at the southern extremity of the Cascade Range, 

 which is largely of volcanic origin, Mt. Jefferson and Mt. Hood stand- 

 ing in the line south of the Columbia River, and Mt. Adams, Mt. 

 Henry, Mt. Rainer, and Mt. Baker, north of the same, the height of 

 no one less than 10,000 feet. Besides, it makes the northern extrem- 

 ity of the Sierra Nevada — which is continuously volcanic at surface 

 from Mt. Shasta to Lassen's Peak, a cone 10,500 feet high on the 

 northern border of Plumas County, and has extensive cappings of 

 lava far south of this point. Mt. Shasta, according to Whitney, is 

 14,440 feet high, 10,000 feet of which rise above the crouching hills 

 at its base. 



(1.) Like most of the great volcanoes of Western America, South 

 as well as North, it has steep sides, the angle of slope averaging 30°. 

 The view here given is from a photograph by Watkins. (2.) Like 

 the others of the Cascade Range it is a volcano that has had great 

 eruptions in the past; but none within recent times, showing a decline 

 in volcanic activity. It appears to be wholly extinct ; but Mt. Hood, 

 of the same range in Oregon, has still some heat and vapors escaping 

 at top. (3.) Its surface lavas are to a large extent basaltic, but 

 trachytic kinds occur toward the base. (Whitney. ) (4.) A wide re- 

 gion of basaltic lavas, from fissure eruptions, surrounds it, and each of 

 the other volcanoes of the series. The accumulated beds in the vicin- 



