BOTTOM DEPOSITS OF THE MURRAY ISLAND REEF. 1 



Dr. Mayer brought from Murray Island six bottom samples, five of 

 which represent a section across the southeast reef along line I, and one is of 

 a sand cast up on the reef 1,700 feet from shore, off the northwest end of the 

 island. The five samples from line I are respectively from the following 

 stations: above high tide, shore end; and 200, 600, 1,200, and 1,600 feet from 

 shore. (See plate 2, of Dr. Mayer's article, for precise location.) He also 

 brought specimens of the calcareous alga, Goniolithon orthoblastum (Heydrich) 

 M. A. Howe, which is so important in the formation of the Lithothamnion 

 ridge; of the limestone which is now elevated 500 to 700 feet above sea-level; 

 and of the lava which, after being pushed upward through the limestone, was 

 extruded over its surface. The lava has been examined by Professor Joseph 

 P. Iddings, according to whom it is a basalt, rich in olivine. There is also 

 some rotten volcanic tuff. The specimen of elevated limestone will be treated 

 as if it were a bottom sample. 



The following are Professor Iddings's notes on the basalt: 



"Four specimens of basalt from Murray Island, Australia, are finely vesicular 

 varieties, black, gray, and reddish brown in color. They are almost aphanitic, but 

 carry minute phenocrysts of olivine less than 0.5 mm. in diameter in 3 specimens, 

 and 1 mm. and less in the gray variety. They may be said to be minophyric and 

 sempatic, there being about equal amounts of minute phenocrysts and groundmass 

 in each variety, and as there is a gradation in the sizes of the phenocrysts from the 

 largest to those that are indistinguishable from the microscopic crystals in the 

 groundmass, their fabric is seriate porphyritic. 



"The mineral composition of the four specimens is quite uniform, and consists 

 of olivine, which forms the most noticeable phenocrysts, less augite, and about the 

 same amount of strongly calcic plagioclase feldspar in microscopic prismoids. 



"The two black varieties have a brown-glass base, with abundant microlites of 

 the minerals just named, besides much magnetite or titaniferous iron oxide in deli- 

 cately dendritic clusters. 



"In the reddish-brown variety the iron oxide and outer portions of the mafic or 

 ferromagnesian minerals are reddened. The gray basalt is apparently holocrystal- 

 line; is rich in olivine and violet-tinted augite, with dendritic magnetite and pris- 

 moids of plagioclase. 



" From the mineral composition it is to be inferred that this basalt is low in silica 

 and the alkalies, and is rich in iron oxide, magnesia, and lime, with normal alumina, 

 and considerable titanium oxide." 2 



The bottom specimens were collected by immersing a bottle and scoop- 

 ing up the material. It is believed that they are fairly representative, for 

 they were taken where about an average amount of coarse material was 

 present and probably very little fine material was washed out as the 



'For a preliminary note, see Carnegie Inst. Wash. Year Book No. 14, p. 220. 



descriptions of the volcanic rocks of Maer Island are given by Haddon, Sollas, and Cole in Trans. Roy. 

 Irish Acad., vol. 30, pp. 432-437, 1894. 



243 



