78 



THE BUILDING OF AN ISLAND. 



acted in the same direction as in the ancient elevation, that is to say, from 

 about north-northeast, and not, as in St. Croix ( ? and in Vieques), from north- 

 by-west. 



The existence of an unusually deep sea channel between the two islands 

 need not present any difficulty in supposing the action, on both sides of the 

 channel, of the same pressure through the earth's crust. When we find, on 

 the chart, depths of i loo, 1500 and even 2500 fathoms, we may at first think 

 of these depths as indicating a great chasm in the earth's crust, so great that 

 it would cut off the possibility of any pressure being exerted across it ; but 

 when we compare these depths with the width of the channel (40 miles) we 

 see that there is not such a great depression after all, and the difficulty disap- 

 pears. (See Fig. 28.) Besides which, we must remember that the deep 

 channel may not have existed when the early upward movements took place. 

 Indeed it is probable that this remarkable channel is of quite recent origin. 



S^C 



Fig. 28. 



V?/^i2*vM..tyL- . 



<J^r. 



The foregoing observations are sufficient to show the connection between 

 the structures of the two islands. There are, however, some differences. 

 Professor Cleve describes the distribution of the rocks of St. Thomas as in 

 three bands, the most southerly, as shown along the coast and in the islets as 

 far west as Little Saba, he describes as felsite (with blue-beach subordinate). 

 The felsite is a verv hard rock, usually of a creamy white colour with much 

 red in it. The south side of Little Saba, as seen from the sea, presents a 

 striking: exhibition of this rock. The Professor remarks in reijard to that 

 islet, that it is commonlv supposed to have been a volcano, which, he says, is 

 not correct, "The rock being of the same kind as at Red Point, a fine-grained 

 felsite, sometimes altered to kaolin." There is very little felsite of this kind in 

 St. Croix. The strip beyond this felsite strip, to the north, is described by 

 Professor Cleve as "blue-beach with felsite subordinate." It is this strip to 

 which the rocks of the town and the central hills already considered belong. 

 Lastly he mentions a band of "stratified metamorphic rock (clay-slate, etc.)" 

 stretching along the north coast from Northside Bay to Coki Point in the 

 east, a strip which may be compared with the clay-slate of St. Croix. The 

 Professor remarks under the head " Blue-beach " (page 40) : " It is very likely 

 that the clay-slate of the Virgin Islands is only an exceedingly fine-grained 

 variety of blue-beach." 



