THE BUILDING OF AN ISLAND. 3 1 



mately when we notice that on the Princess Plain the strata come out in 

 advance of the hills which here border the Central Slope, and dip away under 

 them in a southeasterly or southwesterly direction, while the hills themselves 

 reach heights of 400 to 500 feet (at one point 600 feet). Hence a thickness of 

 at least 600 feet would not be too much to allow for these deposits, an estimate 

 which we shall scarcely consider as too great when we remember that the 

 moderate length of cliff at Cane Garden shows, as ascertained by measure- 

 ment, a thickness of over 200 feet of strata. 



Beach and Reef Limestones. 



We have now studied the limestone formation in the hills, valleys and 

 plains that have been carved out of it, but there still remains a class of lime- 

 stone rocks which, though much younger than those we have been studying, 

 indeed quite recent, arc of very great interest, because in them we see, as it 

 were, the sand of the sea being converted into rock before our eyes. In va- 

 rious places along our sandy shores we find hard, solid beds of rock, which on 

 being broken into show themselves to be nothing else than grains of sand 

 firmly bound together. They are, in fact, made from the shore sand. When 

 these beds are out of the water or exposed at low tide we may conceive that 

 the binding material has been dissolved from the sand itself bv rain-water con- 

 taining carbonic acid gas in solution and has been deposited again amongst the 

 sand grains when the rain-water was afterwards evaporated ; but we find sim- 

 ilar rocks on the reefs, where they are always under water. There is, there- 

 fore, some other source of the carbonic acid than the rain-water, and this is 

 probably to be found in the carbonaceous matter of the animals and seaweeds 

 that are buried in the sand. At the reef at Christiansted the rock is some- 

 times broken off in large pieces with crowbars and brought ashore to be used 

 for building purposes ; hence, blocks of this rock may be seen here and there 

 in walls in different parts of the town, and they show particles of shells or sea- 

 weeds, according to the character of the sand from which they were formed. 

 For the most part they appear to be quite as durable as the stones from the 

 quarries. Sometimes fragments of such rocks from the reef, on being broken 

 through and examined with a lens, show the sections of the sand-grains sur- 

 rounded by a white ring of carbonate of lime, and the same enclosing case is 

 seen around the small pebbles from the Blue-beach formation which are oc- 

 casionally found mixed with the shell sand. At Frederiksted this recent lime- 

 stone rock is not only found along the shore, but in the flat part of the town, 

 w^here it extends itself inland eastwards, so that several streets are built upon 

 it. Here the recent date of the rocks may be determined, not onlv bv the fact 

 that some of the shell fragments have retained their original colours, but also 

 by the presence of a particular foraminiferous shell that is abundant in our sea 

 sands at the present time. (See Fig. 11 on Plate B.) Though so recent a 

 rock, it has already been acted upon bv the weather so far as to be converted 

 near the surface of the ground into a white mass, in which the sand-grains be- 



