FORMATION OF PLAINS. 197 



the island alluded to, the extinction of a species 

 of Heliw, still living in the neighbouring isles, has 

 taken place in consequence. 



The filling-up of sub-marine valleys and ravines 

 with conglomerates and sedimentary deposits 

 gives rise in the end to the formation of plains, 

 which when upheaved form valleys such as those 

 presented by the plains of Macri, Xanthus and 

 Phineka. That of Macri is in continual process 

 of increase, and as we have seen that it has been 

 submitted to frequent and recent changes of 

 level, both through elevation and depression, it 

 affords evidences of phenomena well worthy of 

 the study of geologists. Here we find that the 

 blocking up of the river which flows through the 

 plain, owing to shingle beds and sand heaps, 

 accumulated when the wind is from the south, 

 causes the formation of lagoons and marshes. 

 The water in these is at first salt, but if the 

 bar endure sufficiently long, may become fresh 

 and remain so, and, in the end, be peopled 

 with fresh- water mollusca. At Macri we find 

 such lagoons in all their conditions, filled with 

 myriads of Cerithium mammillatum, a mollusk 

 capable of enduring, without apparent incon- 

 venience, great changes in the quality of its 



