58 THE liUlLUING OF AN ISLAND. 



great heat and pressure from onlinarv limestones, a fact which has been estab- 

 lished b)^ the artificial conversion of chalk into marble under pressure in a blast 

 furnace. It does not appear that our Santa Cruz limestones, even in the 

 older formation, have been jilaccd under like severe conditions with the true 

 marbles. Professor Cleve calls these beds in our older formation merely 

 "compact limestone." In writing of the rocks in the smaller islands around 

 St. Thomas, he says, however, that "Congo Cav is a ridge of beautiful, hard 

 and crystalline limestone or marble, of a bluish gray colour and parallelopipedic. 

 structure." He also mentions crystalline limestone as found in St. John, be-, 

 tween Brown's Bay and Mary's Point, in Tortola, in Ginger Island and in 

 Great Hatch Island. From which it appears that by including our near neigh- 

 bors, islands which indeed, from a geological point of view, are included in our 

 system, we may find illustrations of carbonate of lime in each of its three forms, 

 amorphous , crystalline and crystallized. 



In connection with this verv abundant component of our rocks there is 

 a point which, though somewhat obscure, is of such great interest as to be 

 worth a few lines being devoted to it. The removal of shells and corals im- 

 bedded in the rocks so as to leave hollows still showing their forms, must strike 

 every observer as a very remarkable fact. Why should these objects have 

 been dissolved out and have disappeared, while at the same time the mud in 

 which they were imbedded, and which consisted mostly of similar material 

 in a finely divided state, remains untouched by the dissolving agent? This 

 is a question for the mineralogist to answer, and it is answered by Professor 

 James Dana in his Manual of Geology. 



Carbonate of lime, he tells us, forms two distinct minerals, calc-spar or cal- 

 cite and aragouite. The former mineral, as we have seen, is common in St. 

 Croix, but the latter does not appear to be found here in its crystallized form, 

 though, as we shall presently see, it must be fairly abundant. Aragonite takes its 

 name from the Spanish province of Aragon, where, as well as in some other parts 

 of the world, it is found in great beauty. It is crvstallized in thin prisms, hence 

 sometimes called needle spar, and is rather heavier than calcite. Professor 

 Dana savs that "shells, while consisting generallv of calcium carbonate, often 

 have a large part of the material in the aragonite state; and hence aragonite is 

 present through most uncrystalline limestones." 



Further on, in his notes on " Chemical work," the Professor writes: " If 

 the fossils of a limestone are made of calcite and aragonite [the latter the pris- 

 matic calcium carbonate], the aragonite portion is taken away a fact first re- 

 ported by Sorbv. Shells of the kind referred to are those of the genera Pinna, 

 Mytilus, Spondylus, Patella, Fiisns, Purpiira and Littorina, in which the inner 

 pearly layer- is aragonite, and the outer calcite. The shells of most Gastropods 

 and Cephalopods are aragonite ; and corals, including the millepores, are mainly 

 so; while shells of Rhizopods, Echinoderms and Brachiopods consist of 

 calcite." 



Processor Dana further remarks: " When the minerals aragonite and cal- 



