32 THE Bl'ILDTNCl OV AN ISLAND. 



gin to disappear and which breaks loose in an irregular manner from tiie re- 

 mainder of the rock below. In other words, it is converted, just as in the 

 older limestones, into a white marl. 



Summary. 



With the study of the above described very recent rocks we have com- 

 pleted our general survey of the limestones and marls of the island, and may 

 now pause to summarize the results of our studies before proceeding to give 

 attention to the older or " Blue-beach" formation, on which all the rocks of the 

 later period rest. We have learned, then : 



I. That the limestone formation covers the central, south-central and 

 southwestern parts of our island. 



2. That the rocks are arranged in strata piled over each other to a great 

 thickness. 



3. That the whole mass has been formed in the sea by the gradual accu- 

 mulation of animal and vegetable remains composed mostly of carbonate of 

 lime which the living creatures have extracted from the sea-water. 



4. That at different times during the deposition of the Hmestones and 

 marls, considerable quantities of pebbles and finer debris from the older or 

 "Blue-beach" formation have been washed into the sea and mingled with the 

 accumulations which the sea-creatures have furnished ; from which it may be 

 inferred that land of the older formation was not far off during the deposition 

 of the younger formation. 



5. That great changes in the condition of the rocks have taken place 

 since thev were deposited, the carbonate of lime having in many cases been 

 dissolved and, in part at least, re-deposited in other forms, a change which ac- 

 counts for the hardness of some of the beds, and for the formation of concre- 

 tions, and which has extended also to many of the fossils deposited with the 

 rocks. Another important change is shown by the jointing of many of 

 the beds. 



6. That the whole mass has been raised from the sea-bottom to some 

 height above the sea-level (certainly 600 ft., and in the first instance probably 

 much more). 



7. That the slopes or dips of the rocks show that the elevation has been 

 mostly from the north, but that the dips are also affected by lines of elevation 

 and (if depression which lie in two directions, namcJv from about northeast to 

 southwest, and from about noithwest to southeast. 



8. That as a leading result of these movements tlie central part of the 

 island forms a depression or sort of cross valley filled in with the marl formation, 

 the removal of which from it would leave a sea-channel across the island from 

 north to south, making of the Eastern Triangle and the Northwestern Hills 

 two separate islands. We must remember, however, that this depressed portion 



