VOLCANIC PRODUCTS. 



355 



of basalt. A similar structure is exhibited in all kinds of lavas, and 

 in other rock-masses which have been heated by contact with igneous 

 rocks and gradually cooled. Columns of a dissimilar character arc 

 produced by the unequal cooling of different parts of the stream, so 

 that, if the stream be thick, the lower parts will form stout, vertical 

 columns of great regularity ; while the upper part, cooling less regu- 

 larly, will produce smaller and less regular columns (Fig. 8). Fingal's 



Fig. 8. Section op a Lava- Stream exposed on the Side op the River Ardeche, in the 



Southwest op France. 



Cave, in the Island of Staffa, has been formed in the midst of a lava- 

 stream which has been cooled in this manner. The thick, vertical col- 

 umns, which rise from beneath the level of the sea, are divided by 

 joints and have been broken away by the action of the sea, and a 

 great cavern has been produced, the sides of which are formed by 

 vertical columns, while the roof is made up of smaller and interlacing 

 ones ; and the whole structure bears some resemblance to a Gothic 

 cathedral. The columns formed in cooling vary in size from those of 

 the Shiant Islands, near Skye, which are eight or ten feet in diameter 

 and five hundred feet long, to the minute columns, an inch or two 

 in length and hardly thicker than a needle, of the volcanic glasses. 

 The larger columns are formed in slowly cooling masses. The quan- 

 tity of matter that is ejected from volcanoes in the form of lava is 

 truly enormous. Lava-streams have been described which have flowed 

 for a distance of from fifty to a hundred miles from their source, and 

 which have had a breadth varying from ten to twenty miles ; some are 

 five hundred feet thick, or even thicker. A mass estimated to be equal 

 in bulk to Mont Blanc flowed out in a single eruption of Reykjanes in 

 Iceland, in 1783. In many parts of the earth's surface, among which 

 are tracts in our Rocky Mountain regions, successive lava-sheets have 

 been piled upon one another to the height of several thousand feet, 

 and cover areas of many hundred or even thousand square miles. The 

 marks of the effects of the passage of the hot volcanic matters, with 

 the powerful chemical agents with which they are charged, are left 

 upon the adjoining rocks, where, for a considerable distance from the 

 vent, limestones are converted into statuary marble, sandstones into 



