2 THE BUILDING OF AN ISLAND. 



CHAPTER XI. 



The Relation of the Geology of St. Croix to Geology 



IN General. 



The reader who may wish to extend his studies to the construction and 

 history of the earth's crust as shown in other parts of the world will find valu- 

 able help in such works as Huxley's Physiography and Marr's Introduction to 

 Geology, or, if desirous of following up the subject more fullv, then in Lvell's 

 Principles of Geology and Dana's Manual, but for the sake of others who may 

 wish only a brief statement of how our local geologv stands to geology in gen- 

 eral the present chapter is added. 



The subject appears to fall naturally into three parts; for we mav com- 

 pare the rocks of St. Croix to those of the world in general firstly, in regard 

 to their modes of origin ; next, in regard to the times of their origin relatively 

 considered, or in other words, then- relative ages ; and lastly, we may compare 

 them in regard to their subsequent history. 



I. Origin of the Rocks. 



In our examination of the St. Croix rocks we found that nearlv all of them 

 had been laid down in the sea, and must therefore be classed as aqueous or sed- 

 imentary rocks, but that there were a few which had evidenly been forced up 

 from below in a molten state, and must therefore be classed as ioneotis rocks. 

 Both these great classes of rocks are found in all parts of the world, some of 

 them much like those of our own island, but others very different from them. / 



Igneous Rocks. 



Most geologists have agreed to arrange the igneous rocks in two great 

 classes, namely, the Plutonic and the I'olcanic. 



None of the Plutonic rocks appear ever to have been thrown out b\- yoI- 

 canoes, but they seem to have been formed deep down in the earth's crust, 

 hence their name, from Pluto, the god of the lower regions. The chief mem- 

 bers of this class are the various kinds of granite. The composition of granite 

 varies somewhat, but commonly it consists of the minerals felspar, quartz and 

 mica in distinct crystals. It is a very important rock, and was formerly sup- 

 posed to be the oldest of rocks and to form the base rock of the earth's crust, 

 and, although this view has since been modified by the discovery of granite 

 among the later formations, there can be no doubt that it is verv widely dis- 

 tributed and that it has afforded by its decomposition a considerable part of 

 the materials of the stratified rocks. Some granite is extremely hard and slow 

 to change, but there are other kinds which decompose rather easily; the fel- 

 spar decomposes to a clay, the finest being known as Kaolin, the clay from 

 which fine porcelain is made. When rills pass over the decomposed granite 

 they wash away the clay and redeposit some of it in beds, carrving the rest ulti- 



