SoLLAs& Cole — The Origin of Certain Marbles : a Suggestion. 125 



althougli no trace of organic structure can now be recognized, yet 

 on the whole the appearances are such as might be expected to be 

 presented by a coral-sand rook, which had suffered metamorphic 

 changes. Mac Culloch in his detailed account of this rock refers 

 to its occurrence as an irregular mass, completely surrounded by 

 gneiss; another white limestone occurs in the island similarly 

 disposed. 



It is interesting to speculate on the final result of pressure 

 metamorphism acting on volcanic islands surrounded by their 

 reefs. Thus were the ancient granite masses of Queensland and 

 New Gruinea to approach one another, moving towards the line of 

 weakness which now forms Torres Straits, we may conceive that 

 basic schists in great variety would arise from the rolling out of 

 the cores and superficial deposits of the intervening volcanos, while 

 the associated coral reefs would be converted into irregular masses 

 of structureless limestone, and becoming involved in the surround- 

 ing schists would be irregularly dispersed through them, so as to 

 occur in unexpected and anomalous positions. 



In conclusion we would call attention to an important Paper 

 read in 1876 before the Eoyal Geological Society of Ireland, 

 by Mr. W. L. Green, Minister of Foreign Affairs to the King of 

 the Sandwich Islands.^ Inter alia, he says : — 



" The Hawaiian Islands are more or less surrounded by coral 

 reefs, the island of Hawaii less so than the others, for one reason, 

 because the lava has kept pouring into the sea along most parts of 

 the coast during past centuries, and has not given the coral an 

 opportunity to form to so large an extent as in the other islands. 

 Now it is a fact that wherever the lava runs into the sea, or 

 wherever the waves have an opportunity of breaking against [it] . . . 

 a large quantity of olivine sand is formed. The felspar, the other 

 material of which this lava is mainly composed, gets ground up to 

 powder and disappears — indeed it is almost always in the minutest 

 grains to begin with — whilst the olivine, a much heavier mineral, 

 and in grains from the size of a bean to a pea downwards, forms the 

 main component of the sand of the seashore wherever the sea meets 

 the lava, or else the olivine sand gets more or less mixed up with 

 the coral sand, where the two classes of rock are in close proximity. 



^ Journ. Eoy. Geol. Soc, Ireland, vol. iv., p. 140, 1877. 



