THE BUILDING OF AN ISLAND. 8 



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mately to the sea ; the quartz crystals are separated from the mass and may be 

 redeposited, and if they reach the sea they are rolled on the beach till their 

 edges are removed, and they may then come to form beds of quartz sand on 

 the sea bottom. 



Granite is exposed in great and widely extended masses in many parts of 

 the world and is sometimes found to have been thrust in between layers of 

 stratified rocks, or to have been forced through them as dykes and veins. The 

 present writer has not seen granite in any form in the St. Croix formations, 

 but Professor Cleve mentions granite veins as occurring in several of the islets 

 around St. Thomas and in Tortola. 



Another important rock, which also contributes largely to the supply of 

 material for the stratified rocks, is gneiss. It consists of the same kinds of 

 crystals as those which make up granite, but the crystals are more or less ar- 

 ranged in layers, sometimes with barely a tendency to that form of arrange- 

 ment, so as hardly to be distinguishable from granite. Where, on the other 

 hand, this arrangement is strongly marked it suggests that the rock has first 

 been stratified and afterwards altered by heat. Many geologists, however, are 

 of the opinion that at least some kinds of gneiss, and possibly all kinds, are 

 true igneous rocks that have rearranged their constituents under the action of 

 heat and pressure; that they are, in fact, altered igneous rocks. 



The Volcanic Rocks. Dr. J. W. Spencer, writing about the recent earth- 

 quake in Jamaica, in the magazine "Science," for June 21, 1907, after pointing 

 out that the earthquake could hardlv have been due to volcanic action, since 

 Jamaica is far removed both from the West Indian and the Central American 

 volcanic chains, adds : " Moreover, Jamaica is not volcanic, with only one Pli- 

 ocene volcano upon the northern coast." If there is one extinct volcano in 

 Jamaica it is possible that others may be found in Cuba and Hayti when those 

 islands come to be more thoroughly examined. However that may be, the 

 east and west axis presents a great contrast to that which lies north and south, 

 for, while the former has one volcano and possibly a few more, the latter is full 

 of them. Most of these show no signs of activity and may be regarded as ex- 

 tinct ; others are evidently only slumbering, and in the case of Mont Pelee in 

 Martinique and the volcano of St. Vincent the eruptions of 1902 proved how 

 disastrous their occasional outbursts can be. 



Many of the West Indian volcanoes, perhaps nearly all, have distinct 

 craters, but this does not commonly appear in a distant view. One of these, 

 however, namely, that of the island of St. Eustatius, is a tvpical volcano. A 

 sight of it is very instructive, for not only do we get a hint of the way in which 

 the material far down in the earth's crust may be brought up and piled on the 

 surface, but in the numerous ravines that score its slopes we see how the rains 

 may eat into and remove this material and finally level the great pile which the 

 volcanic force has built up The St. Eustatius volcano is about 2,000 ft. high. 

 From some points of view, both on the island itself and on the neighbouring 

 sea, the crater can plainly be seen to form a hollow, and the volcano is locally 



