LAGOON DEPOSITS 131 



346 feet, or 57 fathoms from the surface. The calcareous 

 alga, Halimeda opuntia, forms the greater part of the first 

 60 feet of material passed through, and is then replaced 

 by a loosely compacted coral rock, which continues 

 down to 245 feet, with occasional intercalations of solid 

 coral reef. The greater part of the floor of the lagoon 

 is at present covered by Halimeda, growing at an un- 

 expectedly rapid rate ; a fortunate observation showed that 

 in one case a mass of this alga, 3 inches in thickness 

 and the same in height, was formed in the course of six 

 weeks. The layer, 60 feet in thickness, below the living 

 superficial film, shows us what it has been able to 

 accomplish in the past, and we are reminded by it of 

 the mode of growth of moss in a peat bog. The hypo- 

 thesis, that the lagoon of an atoll owes its existence to 

 solution, had previously been shown to be inconsistent 

 with observation by the discovery that soundings within 

 a lagoon made at sufficiently long intervals prove a 

 gradual filling up ; and in the living film of Halimeda 

 we now recognise a constructive agent more than com- 

 petent to compete with the destructive effects of solvent 

 water. 



It is a far cry from the low islands of the Pacific Ocean 

 to the mountains of the Tyrol, yet that these noble 

 dolomite peaks once formed parts of ancient atolls, 

 washed by the warm waves of a vanished sea, is an idea 

 which gains in probability with the progress of research. 

 It was first suggested by Baron von Eichthofen, and 

 subsequently maintained by von Mojsisoviks in his great 

 work, " Die Dolomiten Eiffe von Tirol." Subsequently it 

 was shown by Miss Ogilvie (Mrs. Gordon) that the form 

 of these mountain masses was by no means original, but 

 due to subsequent movements of the earth's crust followed 

 by extensive denudation. It was also pointed out that 

 calcareous algae (Gyroporella) contribute largely to their 



