88 THE BUILDING OF AN ISLAND. 



all be classed as mud-stones, and they have their representatives all over the 

 world. 



There are a few other important rocks which are not comparable with 

 anything found in our island. The principal of these is the carbon group, in- 

 cluding coal of various kinds, beds of mineral pitch, and graphite or black-lead, 

 which has been formed from coal by heat and pressure combined. The carbon 

 group has been produced from the vegetation of past ages, the plants having 

 obtained their carbon from the air and their mineral constituents from the soil, 

 and has, therefore, an organic origin. This is not the case with two other re- 

 markable rocks, salt and gypsum, both of which have originated as the result 

 of the evaporation of water containing these substances in solution, and must, 

 therefore, be classed as chemical deposits. 



Enclosed or nearly enclosed seas that are surrounded by hot and rainless 

 countries naturallv tend to become Salter bv excess of evaporation; while the 

 water of the Baltic contains one and one-half per cent, of salt, and that of the 

 Atlantic three per cent., the Mediterranean contains five per cent, and the Red 

 Sea six per cent. If the channel which now connects the Red Sea with the 

 Indian Ocean should ever be closed bv the elevation of the surrounding land 

 and the Suez Isthmus be raised, the Red Sea would decrease in area and in- 

 crease in saltness until at last a great bed of salt would be deposited. The 

 same thing might happen with inland lakes in a country where the climate was 

 changing towards desert conditions. Beds of rock-salt are found in many 

 countries (the nearest to us in St. Croix being Santo Domingo), and it cannot 

 be doubted that they were formed in some such way as above suggested. 



Gvpsum, which is a combination of lime and sulphuric acid, is likewise a 

 deposit from evaporation of inland water areas. It is often found in layers 

 near the beds of rock-salt ; the gvpsum beds in the neghbourhood of Paris, 

 from which the well-known plaster of paris is made, contain, however, fresh- 

 water shells, showing that the rock has been deposited in lakes, the water of 

 which was fresh when the molluscs lived in them. 



There is another important set of rocks that has no true representative 

 here in St. Croix, namely, the sandstones. The sand from which these were 

 formed is entirely unlike the sand of our West Indian seas. As we learned, 

 in the beginning of our observations, our sand is of organic origin, being com- 

 posed of the remains of marine animals and plants, but the sand of the seashore 

 in manv countries is a purely mechanical product, being originally obtained, it 

 would appear, from granite and similar rocks in the wav described under the 

 remarks about granite in this chapter. The sandstones form immense beds in 

 Europe, North America and elsewhere, and are of such importance in some 

 parts that thev have given names to two of the English geological svstems, in 

 which they are abundant. The doubtfully stratified gneisses and the various 

 schists, which will be again referred to in the third section of this chapter, must 

 also be included in rocks not represented in St. Croix. 



We may now leave the question of origin and consider 



