THE BUILDING OF AN ISLAND. 75 



CHAPTER X. 



Relation of the Structure of St. Croix to that of the 

 OTHER West Indian Islands. 



A glance at a map of the West Indies shows that the islands are arranged 

 along two great lines or axes, one lying east and west, the other lying north 

 and south ; the former shutting in the Caribbean Sea on the north, the latter 

 shutting it in on the east. 



The great east and west axis, about 1,200 miles in length, divides in the 

 island of Hayti, in a westerly direction, into two branches, one passing along 

 the northern peninsula of the island and through Cuba, the other passing 

 along the southern peninsula and through Jamaica. From Hayti the undi- 

 vided axis passes eastward through Porto Rico and ends in a group of small 

 islands, of which St. Croix is one. St. Croix belongs, then, to the east and 

 west axis of the West Indies. 



The north and south axis, about 500 miles in length, consists of a curve 

 of volcanic islands, the curve being convex to the open Atlantic. The islands 

 rise rather abruptly from the Caribbean Sea, but have on their eastern side 

 towards the north two wide banks from which rise several non-volcanic islands. 



The obvious points of contrast between these two axes are, that one has 

 for the most part large islands along it, with hardly any volcanos, while the 

 other has in its main line only small islands, all of which have one or more 

 volcanos, mostly extinct, it is true, but some of them, as recent experience 

 has proved, still capable of tremendous occasional activity. 



Our examination of the structure of our island revealed two great periods 

 of elevation, one since the deposition of the limestone formation, and another, 

 an earlier period, following the deposition of the blue-beach rocks. In both 

 cases the position of the layers showed that the principal pressure came from 

 the north ; in the earlier movement it was from north-northeast, and in the later 

 movement it was from north-by-west. We may be sure that there were 

 definite causes, not only for the movements both being from the north, but 

 also for the difference of three points in the direction of the two movements, 

 though we may not be able to discover what the causes were. 



In this connection it is interesting to note that east of Porto Rico the 

 island of Vieques (or Crab Island), nearly as long as St. Croix, lies almost 

 parallel to it, and has, therefore, probably been lifted on the crest of a wave, 

 pushed up by the same movement as that which has lifted our island. Whether 

 there is any geological evidence in Vieques for this theory, the present writer 

 is unable to say. Professor Cleve, after saying that he was in the island only 

 for a few davs and under very unfavourable circumstances, so that he knows 

 very little about its geology, remarks as follows: " Near the shore of Puerto 

 Mula an altered dark green rock is visible, but at a short distance from the 

 coast occurs syenite-like diorite, which seems to be the most important rock 

 of the island, producing by alteration a very fertile soil. The diorite has the 



