THE BUILDING OF AN ISLAND. 59 



cite are present together in a limestone, the first effect of metamorphic action 

 is the conversion of the aragonite into caicite." From which we may infer 

 that in those parts of our Hmestone formation where the fossils have dis- 

 appeared the aragonite has disappeared bv alteration into caicite and has been 

 added to the mass of the rock in this form ; whereas in tiiose parts where the 

 fossils still remain there is some aragonite yet present in the rock, justifying 

 the earlier remark (juotcd, in which aragonite is said to be "present through 

 most uncrvstalline limestones." 



The present writer has not been able to find any rule for the occurrence of 

 limestone fossils in our island in the two forms mentioned namely, as solid 

 fossils and as easts. A clcjse studv of the rocks and the fossils would probably 

 throw some light on the subject and would rcpav the reader who mav have 

 time and opportunity to follow up the subject.'-' 



Quartz is a well-known mineral and occurs abundantly in the older of our 

 two formations. It is easily recognized by its great hardness, in which quality 

 it is exceeded by onlv a few minerals. Commonlv we can see that it has filled 

 up cracks in the rocks, and hence appears as white veins running irregularly 

 through them, while it sometimes forms six-sided crystals in the innermost 

 parts of the veins. 



Flint is nearly the same substance as quartz that is to sav, pure silica, 

 but is not, so far as the writer knows, found in St. Croix. It is in quartz veins 

 that gold is obtained in many parts of the world, but it does not appear that 

 the quartz veins of any part of St. Croix contain that precious metal. 



As already mentioned, the chemical substance which forms quartz is silica, 

 which again is a compound of the elements silicon and oxygen. Quartz has 

 been deposited from silica in solution, and in some parts of the world it has 

 been deposited in large transparent crystals of great purity and beauty ; it is 

 then knowm as rock-crystal. Many beautiful stones used by jewellers, such as 

 amethyst, topaz, cornelian, jasper, etc., are onlv coloured varieties of quartz. 

 Our own island does not, however, appear to contain more than the simplest 

 forms of the mineral. 



Felspai-, of wdiich there are several kinds, is a mineral that mav often be 

 seen as small white crystals in our older rocks; and in the rocks (jf the islet 

 called "Green Cay" it is seen in crystals of unusually large size. This 

 mineral, though not as hard as quartz, is still so hard that it is with difficulty 

 scratched by the point of a knife. Its chief constituents are silica and alumina. 

 Felspar occurs in an uncrystallized form in most of the older stratified rocks 

 of the island and in the igneous rocks. When decomposed it forms clay, 

 sometimes white clq,y, but more commonly broivn clay that is to say, it is col- 

 oured by iron. Hence, it is from the felspar of the rocks that the clay which 

 forms a great part of the soils and subsoils in many parts of the island is 

 derived. 



* The reader who may wish to verify the quotations from Professor Dana's Manual of Geology will 

 find in his inde,^, under Aragonite, references to the pages from which they are taken. 



