THE BUILDINC. OF AN ISLANIl. 35 



Tlir materials of which these rocks are composed have not, like those of 

 the limestone rocks, been extracted from the sea-water, but have been washed 

 down from the land into the sea. If we notice what happens in the harbour 

 at Christiansted after heavy rains we shall understand how thin layers of such 

 deposits mav be laid down. The streams after such rains come down from the 

 hillslopes laden with mud and sand, which settle alona: the shore and for 

 some distance out in the harbour, each intlux producing a thin addition to the 

 deposits. Further light is thrown on the subject by the experience of Martin- 

 ique and St. Vincent after the volcanic eruptions of 1902, when vast quantities 

 of the dust and stones thrown out by the volcanoes were washed down into 

 the sea. As we proceed we shall find some reason to suppose it likely that 

 the older rocks of our Danish Islands, and perhaps the whole chain to which 

 they belong, may have originated from volcanic action, but we must not be too 

 sure of that ; what is quite certain, however, is that these rocks have been 

 formed bv debris brouafht down from the land antl arranged in strata in the 

 sea bottom. 



After having found that the rocks we are now studying are, at all events to 

 a great extent, stratified, it strikes us as very strange when we discover that they 

 are at the same time crystalli7ic, for the materials of which they are composed 

 are not, like carbonate of lime, easily dissolved in water and redeposited in a 

 crystallized state; but it is known that such materials require also a high degree 

 of heat to change their form. It appears, then, that these rocks have 

 been subjected to high temperatures, since their deposition as mud and sand 

 in the ancient seas, and by such high temperatures and the presence of water at 

 the same time, have been so far altered that their stratification for the most part 

 has disappeared, to make itself evident, however, when the weather acts on the 

 exposed surfaces and reveals the stratification anew. In some cases, how- 

 ever, the changes seem to have gone so far that the mass shows no sign 

 whatever of stratification, but resembles those rocks that are found in many 

 parts of the world with forms so plainly showing the action of heat that they 

 are known as ''igneous rocks." 



We do, indeed, find here and there in the older formation of this island 

 some masses of rock that evidently have been forced up from below and are 

 genuine igneous rocks ; but these cases are few. The great mass has 

 undoubtedly been first deposited in sedimentary layers and has afterwards been 

 changed by heat. The name igneo-sedimentary has sometimes been given 

 to such rocks. In a few of the strata the change to the crystalline form has 

 not been carried very far, and these are of great interest, because the records 

 of the past are best preserved in them, as we shall shortly see. 



Having convinced ourselves that the rocks of the Blue-beach" or day 

 formation, in spite of iheir crystallization, are in reality stratified rocks, we may 

 next proceed to study the facts connected with the strata, and the first thing 

 that strikes us is the high angle to which they have been forced up; so that, 

 while the limestone rocks of the younger formation dip at low angles of 10 to 

 15 degrees, the older rocks commonly reach at least 45 degrees. On the 



