336 Captain Puillon-Boblaye on the Tidal and other Zones 



On the shores where the old Hmestones prevail, which princi- 

 pally form the subject of this notice, the submarine table is never 

 more than some j-ards in breadth. Its surface is covered with 

 little rough inequalities, bent back on every side, and fretted in 

 such a manner as only to adhere to the rock in some points. It 

 is, besides, pierced through by deep cavities and sinuosities, di- 

 rected almost always in the plane of the fissures. I have found 

 some localities chiefly on the east side of Magna, where the 

 groove and the submarine table Avhich reattaches itself to its lower 

 part were almost wholly wanting, and of which I was not able 

 to give a reason, the shore being otherwise rocky, steep, and ex- 

 posed to the violence of the sea. I should have been inclined to 

 attribute it to the recent subsidence of the land, and the more so 

 as the sea is very shallow, notwithstanding the steepness of the 

 shore, had not the following observation made me discover that 

 it must be attributed to another cause. There is deposited in 

 these localities a concretionary limestone which encrusts the 

 rocks, however much they may be inclined, even to the superior 

 limit of the tide. Its surface is white, mammillated, and covered 

 with serpulse, with corals and other madrepores. The interior 

 is often formed by a mass of little tubes, as those of serpuljB, 

 but of which the cavities penetrate even to the interior of the 

 crystalline limestone. This deposite which I had not been ac- 

 customed to see formed abundantly, but in those places where 

 the sea is little agitated, could it be here the cause of the pre- 

 servation of the littoral rocks .'' or rather, could not these two ef- 

 fects result from the same cause, the rarity and the feebleness 

 of the winds in the easterly direction * .'' 



" This depot of concretionary limestone occurs wlierever the sea is 

 tranquil, and wherever sands and muds do not change their natures, as in 

 road-steads and mouths of rivers. In the remote and shallow parts of rocky 

 bays, it envelopes many univalve shells to the superior limit of the tide. It 

 is more abundant the calmer and the deeper the sea. Thus, in the road- 

 stead of Navarino, the wreck of the fleet of Ibrahim, raised fi-om a depth of 

 from five to six fathoms, was encrusted with it to a thickness of many milli- 

 metres, after about eighteen months' immersion ; oysters, serpulte, &c. were 

 adhering to it. Leaving out the sands and madrepores, there is a rising of 

 the bottom of nearly five millimetres in two years, which would give ten 

 metres of calcareous deposite during our historical period. We ought also to 

 add, that, in the same place, there is formed enormous muddy and sandy de- 

 posit cs, which have already obstructed two of the entrances of the road-stead. 



