THE BUILDING OF AN ISLAND. 1 5 



into powder, f some of the fragments or the powder again drifting out into 

 deep water. In such ways as these the sand at the sea bottom is continually 

 being increased. Some part, it is true, is again dissolved, as we shall presently 

 see, and part of this may be lost, so far as the material at the sea bottom is 

 concerned, but the bulk of it remains, and so year by year the surface of the 

 great bank is being slowly but certainly raised, and the thickness of the bed of 

 material accumulating at the bottom is being slowly but certainly increased. 



Contributors to the Limestone Formation. 



We may now turn to our marl and limestone rocks to look for further evi- 

 dence than that of the foraminiferous shells as to whether their origin has been 

 similar to that of the sand of our present seas. Without leaving the Anna's 

 Hope rocks we may already go a little farther than the evidence of the fora- 

 miniferous shells will carry us, for w.e will find here an occasional coral, and at 

 least one bed containing the spines of sea-eggs. Here also, in a cutting, on 

 the southeast side of the road and looking southeast, there is found a laver 

 which to some extent departs from that mode of origin, for it contains a 

 number of pebbles and some sand derived from the older formation. It is 

 worth while to pause and study this fact a little more fully, for the same thing 

 may be observed in many other places, and it has an important bearing on the 

 question of the origin of the soils covering the marl formation. The pebbles 

 are nearlv all rounded, as on a beach, an indication that they have been brought 

 down from the ancient land into the sea where the shell-sand was beincf accu- 

 mulated and have been rolled about on its shore before being finally buried. 



ORIGIN. , NUMBER OF GRAINS. TOTAL. 



No. I No. 2 No. 3 



\ Fragments o Nullipore (mo.stly red) 14 34 27 75 ) 



VEGETABLE j ' " Calc. Weed (Halimeda) 7 24 13 44 [- 155 



( " " Corallines 3 23 10 36 ) 



Foraminiferous Shells (whole and fragments) ig 47 34 100" 



Shells, molluscous (mo.stly fragmentary) 6 21 15 42 



ANIMAL . . .-i, Fragments of Echinus Spines 3249 , 



I " " small crustaceous Shells i o i 2 



I Minute Serpula Tubes ' 0314 



I^Fragrhents of Echinus Shell o i o ij 



Agglutinated Grains (same substances) i 6 9 16 



Not determined . S 14 11 33 



Totals 62 175 125 362 



t If we examine with a lens the ordinary sea-sand on an open coast we shall find that most of the sand 

 grains, though obviously fragments of molluscous shells, sea-egg spines, nullipores, or the like, have a 

 rounded form and are beautifully polished, and we can easily see that these peculiarities result form 

 their being ceai5elessly rubbed against each other as the waves come and go on the shore. 



We can further see that a fine powder must in this way be continually rubbed from off these grains, 

 and if we do not find this powder among them, it is because the sorting power of the water carries it away 

 and lodges it farther from the shore. 



After having observed these facts and drawn the plain inferences from them, we need hardly ask for 

 any further explanation of how it is that we find so much finely divided material among the rocks of our 

 limestone and marl formation. Even without allowing for the effects of subsequent changes in the depos- 

 ited materials, the beach history of a part of them would give a sufficient answer. 



