THE BUILDING OF AN ISLAND. 4 1 



tioned. Another curious change in the rock has been the development in it, 

 in some places, of numerous small bodies resembling peas in shape, and gen- 

 erally in size also, though sometimes much smaller. This singular result is 

 shown abundantly in the rocks in Christiansted and the neighbourhood; but 

 mav also be seen in many other places. It has arisen from the tendency of 

 some component of the rock to get together into small masses; it is, in fact, 

 a sort of imperfect crystallization and is technically known as concretion. 

 When the rock is acted on by the weather, the little balls stand out on the 

 surface of the stone, in other cases they have been cut through where natural 

 splits in the rock have occurred, and then they are represented on the surface 

 by small round patches, generally of a lighter colour than the rest of the 

 stone. 



A quarry near the roadside east of Christiansted presents some evidence as 

 to the way in which the process went on. We see on some of the rock faces 

 there very thin bands of paler colour than the rest of the surface, the edges, 

 in fact, of thin layers. In some cases we see these bands broken into short 

 lengths, as if the material which gave the lighter colour had been drawing 

 together in patches; while in other cases again the process has been com- 

 pleted, the material has been collected in little balls all along the line. In 

 the majority of instances, however, the structure in question has arisen in 

 much thicker beds, and then there is frequently very little regularity in its 

 development. 



The structure above described is called by geologists variolitic, a word 

 which refers merely to the variation produced in the stone by the concretionary 

 process. Its occurrence in the rocks is often of great assistance in the deter- 

 mination of the dip. In availing ourselves of its aid for this purpose it is ne- 

 cessary, however, to use a little caution ; for it sometimes happens that the 

 structure tends to follow the Joints of the rock, and if we are not aware of this 

 fact we may be misled into taking the direction of the jointing planes for the 

 direction of the stratification. Generally, however, a careful study of the case 

 will save us from this error. 



In connection with this variolitic structure, another effect of a somewhat 

 similar nature may be noted. A few layers of the older formation may be 

 found in which the tendency of some constituent of the rock to collect towards 

 definite points is shown in a very curious result, namely, in converting the 

 whole bed of rock intt) a number of rounded bodies composed of successive 

 layers, which, under the action of the weather, peel off like the coats of an 

 onion. These rounded bodies are all close together, so that the rock is quite 

 solid, but where the weather acts on it they show themselves, and the coats 

 commence to break away. Such a layer of rock with concretionary forms of 

 this sort, most of them three or four inches across, may be seen in the bed of 

 the stream which passes under the new bridge south of Cornhill. 



Another interesting fact is the splitting of the indurated rock in various 

 directions, so that the fragments generally present several straight sides, the 



