VOLCANOES AND IGNEOUS INTRUSIONS 295 



two days built a cone to a height of about 500 feet. The eruption 

 lasted only a week and has not been renewed since. An examination 

 of the material of the cone showed that most of it was of volcanic 

 rock, but that pieces of Roman pottery, fragments of the surface 

 rock, and marine shells were also present. 



On a plain in Mexico between 2000 and 3000 feet above the sea, covered with fields 

 of sugar and indigo, a fissure opened in 1759, from which rocks were thrown to great 

 heights and about which several cones were built up, the smallest to a height of 300 

 feet and the largest, Jorullo, to that of 1300 feet above the plain. The eruption, which 

 began in June, 1759, ceased in February of the following year. 



Volcanic cones have also been built up from the ocean bottom within recent times. 

 In 181 1 one such (Sabrina) was formed off the Azores, rising to a height of 300 feet 

 above the sea. As it was composed of ash, it was soon washed away by the waves. 

 Many of the great volcanoes of the world, such as Vesuvius, Etna, and Mauna Loa, began 

 as submarine volcanoes many thousands of years ago and built up cones from abyssal 

 depths. 



Classification of Volcanoes. — Volcanoes are usually classified as 

 active, dormant, and extinct. This classification is unsatisfactory, 

 since a volcano which has long been considered to be extinct may 

 become suddenly active, and volcanoes classed as dormant may 

 never again be in eruption. For example, Vesuvius must have been 

 regarded as extinct at the beginning of the Christian era, since it had 

 been inactive so long that its crater was covered with vegetation, yet 

 in a few days in the year 79 one half of its crater was blown off by a 

 series of powerful explosions, and it has been intermittently active 

 ever since. Volcanoes which have not been in eruption during his- 

 toric times are said to be extinct; those which have been active in 

 modern times, but are now inactive, are said to be dormant. All 

 volcanoes may become active after a period of quiet, or may become 

 extinct after a single paroxysm ; such, for example, as that of Monte 

 Nuovo. 



Materials Erupted 



The materials brought to the surface by volcanoes may be classi- 

 fied as gases, solid matter, and lava flows. 



Gases. — The difficulty in collecting gases from the crater of a vol- 

 cano during eruptions renders our knowledge of them rather incom- 

 plete. In fact, whatever information we have has been largely 

 obtained from fumaroles or openings on the flanks of the volcano, 

 and from the crater after eruptions have ceased. 



