18 



LYELL'S ELEMENTS OF GEOLOGY. 



Volcanic Rocks Volcanic Origin of certain Rocks. 



crust, each containing their peculiar fossil animals and plants, 

 which are distinguishable, with few exceptions, from species now 

 living. The mass of some of these strata consists almost entirely 

 of corals, others are made up of shells, others of plants turned 

 into coal, while some are without fossils. In one set of strata 

 the species of fossils are marine ; in another, placed immediately 

 above or below, they as clearly prove that the deposit was form- 

 ed in an estuary or lake. When the student has more fully ex- 

 amined into these appearances, he will become convinced that 

 the time required for the origin of the actual continents must have 

 been far greater than that which is conceded by the theory above 

 alluded to, and that no one universal and sudden conversion of 

 sea into land will account for geological appearances. 



We have now pointed out one great class of rocks, which, 

 however they may vary in mineral composition, colour, grain, 

 or other characters, external and interna!, may nevertheless be 

 grouped together as having a common origin. They have all 

 been formed under water, in the same manner as sand, mud, 

 shingle, banks of shells, coral, and the like, and are characterized 

 by stratification or fossils, or by both. 



Volcanic rochs. — The division of rocks which we may next 

 consider, gjre the volcanic, or those which have been produced, 

 whether in ancient or modern times, not by water, but by the 

 action of fire, or subterranean heat. These rocks are for the 

 most part unstratified, and are devoid of fossils. They are more 

 partially distributed than aqueous form.ations, at least in respect 

 to horizontal extension. Among those parts of Europe where 

 they exhibit characters not to be mistaken, I may mention not 

 only Sicily and the country round Naples, but Auvergne, Velay, 

 and Vivarais, now the departments of Puy de Dome, Haute 

 Loire, and Ardeche, towards the centre and south of France, in 

 which we find several hundred conical hills, having the forms of 

 modern volcanos, with craters more or less perfect on many of 

 their summits. These cones are composed, moreover, of lava, 

 sand, and ashes, similar to those of active volcanos. Streams of 

 lava may sometimes be traced proceeding from the cones into 

 the adjoining valleys, where they choke up the ancient channels 

 of rivers with solid rock, in the same manner as some modern 

 flows of lava in Iceland have been known to do, the rivers either 

 flowing beneath, or cutting out a narrow passage on one side of 

 the lava. Although none of these French volcanos .have been 

 in activity within the period of history or tradition, their forms 

 are often very perfect. Some, however, have been compared to 

 the mere skeletons of volcanos, the rains and torrents having 

 washed their sides, and removed all the loose sand and scoriae, 



