OUTLINES OF GEOLOGY. 



83 



blocks. These falling to the ground may be cemented by- 

 streams of lava, by the finer particles formed into a paste 

 by the water of rains, or by the condensation of the vapour, 

 which is an usual concomitant of volcanic eruptions. 



Aqueous matter not only appears in the form of vapour, 

 but floods of heated water sometimes issue from volcanic 

 craters, or are poured down from the melted snows. There 

 are instances where the matter ejected was mud, and this 

 has been found filled with fish, or charged with matter of 

 vegetable origin. 



The action of subterranean heat changes the character of 

 the rocks exposed to it. Clay and marl are indurated, or 

 even vitrified ; vegetable and animal matter partially chang- 

 ed, or wholly carbonized. Vapours other than that of wa- 

 ter are ejected, and, on cooling, enter into chemical union, 

 forming crystalized minerals, saline or inflammable. 



189. We have seen that volcanic ejection may be prece- 

 ded by the elevation of hills, or even mountains. Earth- 

 quakes, even where they are not followed by eruptions, may 

 produce elevations of the land. The most remarkable in- 

 stance of this kind is to be found on the coast of Chili, 

 where, under the eyes of the existing generation, an extent 

 of many miles of beach was raised 20 feet above its former 

 level. Other beaches were, on examination, found on the 

 same coast, which must have been raised in a similar man- 

 ner, at former periods. As earthquakes are a phenomenon 

 inseparable from volcanic action, we may assign to the agen- 

 cy of subterranean heat this remarkable phenomenon. 

 With it may be classed, although the connection is obscure, 

 the slow and gradual elevation, which it is now admitted is 

 taking place on the Baltic coast of Sweden. 



190. In examining the orders of rocks we have classed 

 as ancient, no doubt can remain that the several species of 

 action which we find in operation at the present day, have 



