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XIII. 



THE OEIGIN OF CEETAIN MAEBLES : A SUGGESTION. 

 By PEOFESSOES SOLLAS AND COLE. 



[Read May 20, 1891.] 



Amongst the interesting collection of rocks brought home by 

 Professor Haddon from Torres Straits are some fragments of 

 wind-blown coral sand rock from the east side of Mer, one of the 

 Murray Islands. They have a deceptively oolitic appearance ; and 

 the majority of the grains being of a red colour give a prevailing 

 warm tint to the stone, and thus render more conspicuous by contrast 

 a number of dark green, worn, and rounded crystals of augite, which 

 are scattered irregularly through it. The appearance of this hand- 

 some rock is sufficiently striking, but it gains greatly in interest 

 from its suggestive resemblance to the famous Tiree marble, 

 wherein likewise green grains of pyroxene are set in a flesh- 

 coloured matrix of altered limestone. The comparison is confirmed 

 and enhanced by an examination of thin slices. In the recent 

 limestone the calcareous grains are found, as so commonly happens 

 with these coral sand rocks, to consist of rounded fragments of 

 calcareous algse and worn tests of various species of foraminifera. 

 Mingled with these are more or less rounded crystals not only of 

 green augite, but also of olivine, felspar, and a finely crystalline 

 glassy basalt. In the Tiree marble the green grains of pyroxene 

 (salite) show beautifully rounded outlines, and are sharply sepa- 

 rated from the surrounding matrix, into which they show no 

 tendency to pass. Crystals of felspar are also present, some fairly 

 fresh, others, and these are the majority, corroded and almost 

 entirely replaced by calcite, only the thin outer skin of the felspar 

 preserving a fresh appearance. In some few cases, fragments of 

 felspar partially penetrated by salite are met with. The calcareous - 

 matrix is finely granular, possibly dolomitic, but blotched and 

 spotted by badly-defined larger crystalline individuals of calcite, 

 the outlines of which are sometimes obscurely rounded. Thus 



