34 THE BUILDING OF AN ISLAND. 



CHAPTER IV. 



TiiK "Blue-beach" or Indurated Clav Formation. 



As already mentioned, the " Blue-beach" formation is found throughout 

 the Eastern Triangle and the hills of the Western Oblong; in fact it occupies 

 all parts of the island not occupied by the limestone and marl group. Not 

 only so, but it passes under those rocks, and is the base on which they all rest. 

 It is therefore the principal, as well as the older formation. 



In comparing, at the beginning of our studies, the rocks of the two groups, 

 it was pointed out that by the simple experiment of dropping a fragment of 

 each into water containing a little nitric or muriatic acid it can be seen at once 

 that there is some great difference in their composition, for while the frag- 

 ment from the younger group effervesces and at last almost disappears, the 

 fragment from the older group is scarcely, if at all, affected. For a proper 

 account of their composition, however, we must look to the chemists, and 

 they tell us that these older rocks are mainly composed of alumina and silica, 

 the substances which in combination form clay, silica by itself forming flint, 

 quartz and similar substances. 



In 18^9 an American geologist, Prof. S. Hovev, published some observa- 

 tions on the geology of St. Croix. He distinguishes the two formations of 

 the island, and calls the older one the Clay Formation. He also mentions it 

 as indurated clay, a better name, since the rocks, though like clay in their com- 

 position, have been made intensely hard by processes to which they have been 

 subjected since they were deposited. He speaks of their composition as va- 

 rious, in some cases silica, in others alumina, predominating. He also states 

 that some of the strata near Mt. Victory resemble slates. 



In 1868 a Swedish geologist. Prof. P. T. Cleve, visited the northeastern 

 West Indian Islands and gave an account of their geology. In describing the 

 ofeoloarv of the Danish Islands he restricts the local name " blue-beach " to a 

 conglomerate which is abundant in St. Thomas, and he speaks of some of our 

 Santa Cruz rocks as resembling the said "blue-beach conglomerate." Locally, 

 however, the term "blue-beach" is applied to all the hard crystalline rocks of 

 a blue or green colour, and the term "rotten-stone" to the same rock when 

 broken up by weathering, as we see it in the so-called gravel-pits. Prof. 

 Cleve also speaks of clay-slate as abundant east of Christiansted and northeast 

 of Frederiksted, and gives many other interesting particulars, to some of which 

 we may refer later. 



The amateur study of the older rocks is at first rather discouraging, for 

 they often show no sign of any arrangement, but look like mere masses of 

 rusty material, presenting no hint as to how their study is to be undertaken. 

 We find out, however, as we go on, that they give in many places clear in- 

 dications of regular stratification. In many of them these indications consist 

 merely of ribbon-like markings on their outer surface, but sometimes the lavers 

 are also divided along surfaces corresponding to those markings, so as to leave 

 no possible doubt of their stratified arrangement. 



