THE BUILDING OF AN ISLAND. 93 



the nautilus in form, the shells of which arc known as Ammonites. Some of 

 these ammonites are of large size, and the writer remembers seeing near Port- 

 land, in Dorsetshire, manv of them used along with the slabs of Oolite lime- 

 stone to form stone fences between the fields. 



The lower beds of the cretaceous system are not chalk, but sands and 

 clays, and their fossils have considerable resemblance to those of the Jurassic 

 period. The chalk beds themselves contain numerous shells and sea-eggs, but 

 are chiefly characterized by being formed largely of foraminiferous shells and 

 their fragments. The flints, which lie in sheets in the chalk and show as 

 dotted lines in the cliffs, are concretions of silica, the source of which has been 

 the silica extracted by marine creatures, such as sponges, from sea-water. 

 This may be illustrated by the sponges of the present day, many of which 

 contain flinty spikes such as may sometimes be seen in the sand from the sea 

 bottom, say, for example, in Christiansted Harbour. 



The fossils of the Tertiary Period show us the gradual approach to the 

 life of the globe as it now exists through numerous remains of both land 

 and sea creatures and various plants. The appearance of man on the earth is 

 a recent event, speaking in geological terms, but a very ancient one if we use 

 the language of human history. 



The above remarks may perhaps serve to give some faint idea of the great 

 extent of the study opened up by the fossils which the various strata of the 

 earth's crust have yielded and continue to yield; a study, the difficulty of which 

 is increased by the many breaks that must necessarily come in the record and 

 by the necessity of considering the climate and other conditions, not only of 

 the various periods, but of the difl^erent places to which the fossils respectively 

 belong. But we must now return to the main purpose of this section, namely, 

 to answer the question, what are the ages of the rocks of St. Croix in relation 

 to those of the earth's crust in general, where must our formations be entered 

 in such tables as have been quoted ? The answer must be left to the geologists, 

 and we find that Professor Cleve, judging from the fossils collected here and 

 in the neighbouring islands, puts down our Marl and Limestone Formation as 

 of Miocene age and our Bluebeach Formation as Cretaceous ; from which it will 

 be seen that, notwithstanding the great periods of time that must have passed 

 during the deposition, the subsequent movements and the sculpturing of our 

 St Croix formations, thev are comparatively young in the world's geological 

 history, and we begin to have a dim perception of what a grand and marvellous 

 work the creation of this earth of ours has been. 



3. History of the Rocks Subsequent to their Origin. 



xA.!! the various changes in the character of the rocks which we found to 

 have taken place in St. Croix, such as hardening, jointing, cleavage, formation 

 of concretions, alteration by heat, and so on, are known among rocks else- 

 where ; but all rocks have not been submitted to these changes, a striking ex- 

 ample of which is found in the London Clay, a deposit of about 500 feet in 



