88 



Couthouy on Coral Formations 



SECTION OF A RAVINE CLOSED AT BOTH EXTREMITIES, IN ONE OF THE 

 VALLIES RUNNING FROM THE COAST TO THE BASE OF WARITIVA, IN 

 TAHITI. 



Co 



a. a. a. Lonejitudinal section of the ravine. 



b. b. Steep lateral ridge, rising to a height of two thousand feet. 



The mountains in outline, are from five lo eight thousand feet high. 



It is difficult, satisfactorily, to account for these singular 

 chasms. That at Waihiria may indeed have been produced 

 by a landslip blocking up the valley, but the one last men- 

 tioned would rather appear, were it not for the perpendicular- 

 ity of its terminal walls, to have arisen from a confluence of 

 the two great lava streams forming the lateral ridges, as the 

 rocks are on all sides in a normal position and of uniform 

 structure. Or they may be owing to a sudden sinking in of 

 the crust at the time when the subterranean fires were in ac- 

 tivity. Pits, very similar to them, but of less extent, are of 

 quite frequent occurrence on the black ledge of lava surround- 

 ing the crater of Kilauea. The second class of ravines may, 

 I think, generally be referred to ancient craters, one side of 

 which has been rent apart by earthquakes. In their situa- 

 tions and outline, in the uniform perpendicularity of their 

 parietesj and the sub-columnar structure of the lava compo- 

 sing them, they correspond exactly to the craters of the table 

 land of Mauna Loa. In fact, not many years ago, during a 

 sharp earthquake, a similar outlet, since fiUtjd up near its com- 

 mencement by subsequent overflowings of the lava, was 

 produced on the S. W. side of the great crater of Kilauea or 



