lOO . THE BUILDING OF AN ISLAND. 



material in question. At all events, the strata we now have were laid down 

 at the sea bottom, and they reached a thickness so great that we must suppose 

 that the land from which they were being formed was sinking while the accu- 

 mulation was in progress. 



What depth of material was lodged in that ancient sea? If we could get 

 anv reliable ajiproximation to the total thickness of the beds in the older 

 formation, as we can do for the marl and limestone formation, it would be easy 

 to answer the question, so far, at least, as regards the part now remaining; but 

 we do not see the base on which the rocks rest, and the strata, as we have 

 seen, have been very much disturbed. Professor Cleve, writing of St. Thomas, 

 says: "The blue-beach seems to me to be at least 2,000 metres (about 6,500 

 feet) thick." This probably excludes the felsite of the south and the clay- 

 slates of the north, and if so the total thickness of the St. Thomas strata would 

 presumably be much greater than that stated. If in St. Croix we take the 

 distance between Lower Concordia and Mount Washington, across the southern 

 side of the northwestern anticlinal and assume a high angle of dip throughout, 

 we should get a thickness of at least 12,000 feet; but on the one hand we 

 cannot be sure that at Concordia we have arrived at the southern limit of the 

 southerly dipping beds, neither do we know the depth to which the strata 

 reach at the axis of the anticlinal, and on the other hand we cannot be sure of 

 the absence of minor folds, the presence of which might considerably reduce 

 the estimate. It seems, however, to be safe to conclude that the total thick- 

 ness of the clay formation is very great and is to be reckoned by thousands of 

 feet instead of bv hundreds as in the limestone formation. While this great 

 mass of clays was being piled up, the sea added on its own account occasional 

 small contributions of limestone. 



After the deposition of the great thickness of clays and its few associated 

 limestones, the accumulated beds were forced up above the sea and compressed 

 into the numerous parallel ridges and hollows which we have found them now 

 to reveal, and this process probably extended through a very long period of 

 time. Other processes, most likely at the same time, and perhaps from the 

 same causes, went on, the strata being to a great extent made crystalline and 

 otherwise altered to the forms in which we now see them. Dvkes and masses 

 of igneous rocks were, probably about the same time, forced into them ; but 

 that this invasion of molten matter ever went so far as to produce eruptions 

 ending in the formation of volcanoes there appears to be no evidence to show. 



Thus a new land was formed out of the waste of the old ; but no sooner 

 was it pushed above the sea level than the sea began to attack its edges, and 

 the atmosphere and the rains began to lower its surface ; and continuing 

 through long ages, these forces wore down the slopes of its ridges, first into 

 hill and dale, and then, in part at least, into plains, just as they had done with 

 the ancestral land. 



Then comes another swing of the pendulum, the worn down land begins 

 to subside, and on its plains, now under the sea, great thicknesses of sea sand, 



