94 THE BUILDING OF AN ISLAND. 



thickness, above which, on some overlying gravels and clays, the citv of Lon- 

 don stands ; this great deposit has remained a mere stiff clav, not entirely free, 

 however, from changes, for layers of concretionary blocks of hydraulic lime- 

 stone, the blocks known as septaria, have been formed in it. 



The term " metamorphism " (change of form) is used to indicate all im- 

 portant changes that have taken place in rocks since their deposition ; but the 

 name metamorphic rocks is commonly restricted to clay-slates, marbles, gneiss, 

 schists and other rocks that have been laid down as sediments and subse- 

 quently altered by heat. Some of these, as already noted, we have in the 

 Danish West Indian Islands; others are not represented here. 6";/m.f, already 

 mentioned under igneous rocks, is a very abundant rock, consisting of the 

 same crystals as granite, "but the materials showing signs of a stratified arrange- 

 ment. In some masses of gneiss the tendencv is so sliijht that ceolosfists have 

 supposed this peculiar arrangement to have resulted, like the cleavage in slates, 

 from pressure ; in other cases the linear arrangement is so conspicuous that 

 the rock has been supposed to have been originally a stratified deposit from 

 granite, which has been subsequently altered and hardened by heat. The 

 Schists are rocks dividing naturally into irregular layers (the division is implied 

 by the name) and are apparently altered sedimentarv deposits derived from 

 granite rocks. Shales are hardened muds, frequently showing a similar tend- 

 ency to division. 



In all parts of the world the sedimentary rocks are found to have under- 

 gone similar upliftings, bendings, denudations and depressions as we find re- 

 corded in the rocks of St. Croix. Even the summits of the highest mountains 

 frequently show stratified rocks, just as do those of the lower hills and the 

 plains. Evidence has been adduced to show that the lifting and lowering of 

 certain parts of the earth have gone on, as in the geological past so also in 

 historic times. Earthquakes, even in the present day, are sometimes attended 

 with changes in the level of a country, and are probably the effects of such 

 changes. This has been particularly the case with the shores of the South 

 American countries bordering the Pacific ; but the most striking illustration 

 which the present writer has met with is recorded in the June number (1907) 

 of the ' Outlook," in an article headed "An Alaskan Wonderplace," by Oscar 

 von Engeln. Here, the traveller tells of the grand mountains and glaciers in 

 the vicinitv of Yukutat Bay, and after describing the way in which a great 

 glacier grinds down the rocks over which it passes, and how the material it re- 

 moves is carried off by the stream issuing from beneath the foot of the 

 glacier and by it is spread out over the adjoining i)lain, he continues thus: 



."If the efforts which the glacial streams are making to counteract the 

 destructive erosion of the ice are termed herculean, then titanic must be the 

 word applied to the force which, in a single upthrust, lifted the land surface 

 around Yukutat Bay, mountains and all, forty feet higher above the sea level 

 than they had been previously. Never before in the historv of geological 

 science has a recent u])lifl nf such magnitude been recorded. Moreover, tlie 



