THE BUILDING OF AN ISLAND. 99 



CHAPTER XII. 



Conclusion. 



While it seems desirable, in entering on the study of the geological forma- 

 tions of our island, to begin with the younger set of rocks and trace the story 

 backwards, it may be instructive, in summarizing the results of our observa- 

 tions, to take the opposite course, and, as far as possible, note the leading 

 events and conditions in their natural sequence. 



As a preliminary, we must remember that the crystalline structure 

 prevalent in our older formation has been induced in the strata since the 

 materials of which they are composed were deposited, and similarly that 

 the dykes of igneous rock which we find cutting them through have been 

 intruded, so that in the first instance we have to confine our thoughts simply 

 to their deposition. 



To begin with, we have, then, to picture to ourselves as existing here long 

 ages ago, perhaps about the time that the chalk of Europe was being deposited, 

 a sea bottom to receive the beds and an adjacent land from which the material 

 to form them was obtained. That the land was near by is shown by the 

 occurrence of conglomerates, and in a striking way by the layer of well water- 

 worn pebbles previously mentioned as seen in Buck Island, a layer which must 

 either have formed a part of a shinglv beach or have been put down in shallow 

 water not far from it. The proximity of the land for at least the greater part 

 of the time during which the materials were being deposited, is further shown 

 by the frequency of the occurrence of ribbon-like markings in the stone, 

 markings which indicate successive deposits of thin layers and point to oft 

 repeated invasions of sediment from the shore, and it is also shown by the very 

 numerous beds that have reached thicknesses of a few inches only and may 

 well have been put down not far from the land. The thick beds of fine- 

 grained slate, such as those we see near Mt. Victory, must have been formed 

 farther out, in places to which the currents could bear the finest drift only; 

 yet a distance of one or two miles would, in most cases, be sufficient to secure 

 the needful conditions, though it may, of course, have been very much 

 greater. 



The conception of another land still farther back in time, a land which we 

 have pictured as being destroyed in the making of the older portion of our 

 island, arouses our interest. What sort of a land was it? We see in our own 

 rocks part at least of the stuff of which it was built, and we perceive that it 

 had a sea beach, in some parts bordering shallow seas, and it had streams that 

 ran down from its hills and valleys and across its plains. Was it a volcanic 

 land? The finding in the strata of St. Thomas of scoriaceous fragments, as 

 described by Prof. Cleve, proves that it was, and it may be that the volcanoes 

 furnished a eood deal of the material from which our rocks were formed. On 

 the other hand, the volcanic outbursts may have taken place through non- 

 volcanic strata, and the latter may have supplied the greater part of the 



