OCR Text |
Show ISLAND LIFE. [rAR'r r. oceanic mud should have been at once seized upon, and the conclusion arrived at that chalk is a deep-sea oceanic formation exactly analogous to that which has been shown to cover large areas of the Atlantic, Pacific, and Southern oceans. But there are several objections to this view which seem fatal to its acceptance. In the first place, no specimens of Globigerinaooze from the deep ocean-bed yet examined agree even approximately with chalk in chemical composition, only containing from 44 to 79 per cent. of carbonate of lime, with from 5 to 11 per cent. of silica, and from 8 to 33 per cent. of alumina and oxide of iron. 1 Chalk, on the other hand, contains usually from 94 to 99 per cent. of carbonate of lime, and a very minute quantity of alumina and silica. This large proportion of carbonate of lime implies some other source of this mineral, and it is probably to be found in the excessively fine mud produced by the decomposition and denudation of coral reefs. Mr. Dana, the geologist of the United States Exploring Exp~dition, found in the elevated coral reef of Oahu, one of the Sandwich Islands, a deposit closely resembling chalk in colour, texture, &c.; w bile in several growing reefs a similar formation of modern chalk undistinguishable from the ancient, was observed.2 Sir Charles 1 Sir W. Thomson, Voyage of Challenger, Vol. II., p. 374. 2 The following is the analysis of the chalk at Oahu:- Carbonate of Lime ... : ........................ 92·800 per cent. Carbonate of Magnesia....................... 2 ·385 , Alumina......................................... 0·250 , Oxide of Iron................................... 0·543 , Silica............................................. 0·750 , Phosphoric Acid and Fluorine.............. 2·113 ,, Water and loss................................ 1·148 , This chalk consists simply of comminuted corals and shells of the reef. It has been examined microscopically and found to be destitute of the minute organisms abounding in the chalk of England. (Geology of the United States Expl01·ing Expedition, p. 150.) This. absence of Globigerince is a local phenomenon. They are quite absent m the Arafura Sea, and no Globigerina-ooze was found in any of the enclosed seas of the Pacific, but with these exceptions the Globigerince "are r?ally found ~11 over the bottom of the ocean." (Murray on Oceanic Depos~ts-Proceedzngs of Royal fJociety, Vol. XXIV., p. 523.) The above analysis shows a far closer resemblance to chalk than that . of the Globigm·ina-ooze of the Atlantic, four specimens of which given by CHAP. n.J GEOGRAPHICAL AND GEOLOGICAL CHANGES. 89 Lyell well remarks that the pure calcareous mud produced by the decomposition of the shelly coverings of mollusca and zoophytes would be much lighter than argillaceous or arenaceous mud, and being thus transporteu to greater distances would be completely separated from all impurities. Now the Globigerinre have been shown by the Ohallen,qe1· explorations to abound in all moderately warm seas; living both at the surface, at various depths in the water, and at the bottom. It was long thought that they were surface-dwellers only, and that their dead tests sank to the bottom, producing the Globigerina-ooze in those areas where other deposits were absent or scanty. But the examination of the whole of the dredgings and surface-gatherings of the Challenger by Sir W. Thomson (Voyage of the Challenge?., Vol. II. Appendix, pp. 374- 376, Nos. 9, 10, 11 and 12) from the mid-A~lantic, show the following proportions:- Carbonate of Lime ................ 43•93 to 79·17 per cent. Carbonate of Magnesia .......... 1·40 to 2·58 Alumina and Oxide of Iron ..... 6·00? to 32•98 " Silica .................................. 4·60 to 11·23 " " In addition to the above there is a quantity of insoluble residue consistillg of small particles of sanidine, augite, hornblende, and magnetite, supposed to be the product of volcanic dust or ashes carried either in the air or by ocean currents. This volcanic matter amounts to from 4·GO to 8·33 per cent. of the Globigerina-ooze of the mid-Atlantic, where it seems to be always present ; and the small proportion of similar matter in true chalk is another proof that its origin is different, and that it was deposited far more rapidly than the oceanic ooze. The following analysis of chalk by Mr. D. Forbes will show the difference between the two formations :- Carbonate of Lime .................... . Carbonate of Magnesia.......... . . .. Alumina and Phosphoric Acid .... .. Chloride of Sodium .................. .. Insoluble debris ......................... . Grey Chalk, Folkes tone. 94·09 0·31 a trace 1·29 3•61 Wllite Chalk, ShoreltMn. 98·40 0·08 0·42 1·10 (From Qua1·tm·ly Jou1·nal of the Geological Society, Vol. XXVII.) The large proportion of carbonate of lime, and the very small quantity of silica, alumina, and insoluble debris, at once distinguish tnle chalk from the Globige1·ina-ooze of the deep ocean bed . |