86 THE BUILDING OF AN ISLAND. 



from below, giving rise to dvkes. It may be asked how such facts as these 

 last mentioned can be known? They are learned from old craters antl from 

 the valleys which are sometimes cut out by streams in volcanic districts and 

 reveal the structure of the mountain slopes; particularlv has this been the case 

 with the ancient and long extinct volcanoes of Auvergne in Central France, 

 where, although the cones and craters have been preserved, the mountain sides 

 have been scored by streams, whose eroding action has opened up to the 

 student of volcanoes many of the secrets of those wonderful natural agents. 



As already remarked, there are no evidences of the existence of volcanoes 

 in St. Croix; if there have been any they have been long since swept away, 

 yet, as we have seen, there are abundant evidences of the action of heat, by 

 which most of the older sedimentary rocks have been greatly altered and by 

 which dykes and masses of igneous rock have been thrust in among the stratified 

 rocks. 



These igneous rocks of St. Croix, not being plainly of volcanic origin, 

 would be placed bv some geologists in a third class, intermediate between the 

 Plutonic and the Volcanic, but as very similar rocks can be shown in some 

 parts of the world to have been formed below volcanoes, it seems hardly, nec- 

 essary, so far as the rocks themselves are concerned, to make the distinction. 

 If it is made, the igneous rocks of St. Croix mav be described as trap rocks, 

 and the question of their possible connection, or the connection of some of 

 them, with ancient volcanoes, may be left open. 



Geologists have found that volcanoes have existed from very far back in 

 the history of the earth's crust. Those of the West Indian north and south 

 axis appear, however, to have originated in a quite recent geological age, the 

 around for which conclusion will be mentioned later. 



The origin of the heat which causes volcanic activity, as well as such 

 alterations and such intrusions of trap as we find in St. Croix, has been much 

 discussed. Some have supposed that the earth's crust rests on a globe of 

 molten rock, which here and there oozes upwards and sometimes comes out 

 on the surface ; but it has been shown that this is impossible, since a crust so 

 situated would be racked to pieces by tidal movements in the fiery ocean 

 below. Others have supposed that, although the earth has so far cooled down 

 that there is no such central ocean of fire, there are local patches of molten 

 rock, from which the supplv in the volcanic vents is derived. This view is 

 still held by some geologists, while others maintain that the heat is developed 

 bv chemical changes in the rocks and by alterations of pressure as the earth's 

 crust contracts, differences of opinion which show us that while the effects are 

 known the causes are still obscure. 



The Sedimentary Rocks 



Having seen the relation in which the few igneous rocks of our island 

 stand to the class generally, we may now consider the relation as it concerns 

 the orisin of the sedimcnlarv rocks. It will be recalled that these rocks in St. 



