THE BUILDING OF AN ISLAND. 87 



.Croix belong to two very different sets, the younger set being of organic 

 origin, that is to say, derived from hving creatures whicli had extracted the 

 lime of the hard parts of their structure from the sea-water ; the older set 

 being of mechanical origin, the material of its beds having been washed down 

 from the land into the sea. Both these two great classes of sedimentary I'ocks, 

 the organic and the inorganic, have their representatives all the world over. 

 Let us first compare some of these with our own and afterwards see whether 

 there are not other great groups of sedimentary rocks, which are not at all 

 represented in St. Croix. 



The Lime Rocks. 



Varieties belonging to this class are common all over the world. One of 

 the best known is chalk, frequently, though not always, a soft rock. It is 

 mostly composed of minute shells and fragments of these, and many of its 

 beds contain layers of the siliceous concretions known as flints. Flints, how- 

 ever, are not restricted to the chalk, for, as already mentioned, they are to be 

 found in some of the limestone beds in the island of Antigua, though there do 

 not appear to be any in those of St. Croix. 



All limestones are probably of organic origin, though this cannot be said 

 to be quite certain. Many that show no fossils when cursorily examined, are 

 found on further examination to contain them, while some limestones are full 

 of them ; for example, the encrinitc beds of the English carboniferous lime- 

 stone, beds which are crowded with the stems of feather-stars, something like 

 those which are occasionally brought up by our fishermen from the deepest 

 parts of the banks, or again the Purbeck limestone, which has originated in 

 fresh water and is full of the shells of fresh-water snails. Both these lime- 

 stones are compact enough to take a polish, arc consequently often used for 

 ornamental purposes, and are sometimes spoken of as marbles. That name, 

 however, ought properly to be applied only to those limestones which have 

 been so much altered by heat and pressure that their origin is completely 

 obscured ; such are, for example, the marbles of Greece and Italy. As already 

 mentioned, there do not appear to be any of these marbles or crystalline lime- 

 stones in St. Croix, although some are found in the neighbouring Virgin 

 Islands. In some parts of the world considerable beds of limestone are found 

 in which magnesia in considerable quantities, occasionally equal to nearly half 

 of the rock, is found. Such limestone is known as dolomite or magnesian lime- 

 stone, and is supposed to have been formed in inland seas by deposition, as 

 the result, in part at all events, of evaporation. It may, therefore, be classed 

 as partly a chemical deposit. The last limestone that need be mentioned here 

 is Oolite, or "roe-stone," a hmestone made up of tiny balls of carbonate of 

 lime all bound together as a firm rock, which, when broken through, presents 

 a surface resembling the roe of a fish. 



Clay Rocks. 

 The indurated clays and slates of our island may in regard to their origin 



