346 



Henry Woodward — On Volcanos. 



lately as in 1866. We may also compare the Island of Thera "with 

 that of St. Paul. 



Graham's Island is another illustration of a sub-marine volcano. 

 This island made its appearance July 10th, 1831, off the South-west 

 coast of Sicily. It was then 12 feet above water ; on the 10th of 

 August it had attained the height of 800 feet ; but being composed 

 of loose scorise and cinders, and having no lava to solidify its cone, 

 it began to be rapidly destroyed and washed away by the sea, and 

 has now entirely disappeared. 



The products of volcanic eruptions may be roughly divided into 

 solid, liquid, and gaseous. Of the solid matter thrown out from 

 volcanic vents may be enumerated " Volcanic Bombs," "Scorias" (or 

 cinders), "Lapilli" (or gravel), " Pozzuolana" (or sand), "Pumice," 

 and "Ceneri" (ashes). 



Of the liquid matter poured out, either from the crater itself or 

 from rents in its sides, we have the substance called " Lava," which, 

 when solidified, may be either Basalt, Obsidian, or Vesicular Lava, 

 depending upon the condition under which it cools, or the tempera- 

 ture at which it is ejected. 



Basalt is lava solidified under pressure ; it frequently presents 

 the condition of "Columnar Basalt," as in the Auvergne, Stafifa, 

 Antrim (Fig. 8), New Zealand, etc. 



Fig. 8. — Basaltic columns from the Giant's Causeway. In the apartments of the 

 Geological Society, Somerset House. 



The most glassy and semi-translucent varieties of lava are known 

 by the name of "Obsidian," or "Volcanic Glass," and occur 

 abundantly in Iceland, the Lipari Islands, Mount Ararat, and Mexico. 



Large tracts in Mexico (called Malpais) are covered with " Obsi- 

 dian," and of it — as did our prehistoric ancestors from the chalk-flints 

 in Britain — the worshippers of the sun manufactured their knives, etc., 

 which were still in use at the time of the Spanish conquest (1518). 



