78 



SCIENCE-GOSSIP. 



A HISTORY OF CHALK. 



By Edward A. Martin, F.G.S. 



{Co7itinned/ro]n page 43. ) 



T X THEN we pass beyond the range of the oozes, to 

 ' ^ those paits of the ofiean explored by the 

 " Challenger " where there is a depth of over two-and- 

 a-half miles, we find that, although these same micro- 

 scopic organisms live in abvmdance at the surface, yet 

 none of their tests reach the bottom. The carbonate 

 of lime of which their tests are composed is removed 

 chemically in the greater depths of the ocean, and 

 probably becomes converted into soluble bicarbonate 

 of lime. At this tremendous depth we find that the 

 deep-sea deposits consist chiefly of red and chocolate 

 clays, to the exclusion of microscopic Protozoans. 

 These clays are believed by those who have studied the 

 subject, to have been derived from material erupted 

 by volcanoes ; which, owing to its rapid cooling in 

 air, has assumed the form of vesicular pumice-stone. 

 Large quantities of this stone, in a "water-logged" 

 condition, were dredged from the red clay areas. 

 The clay itself has apparently been derived from the 

 action of carbonic acid in the water upon the 

 pumice-stone, decomposing its felspar, and forming 

 clay or hydrated silicate of alumina. Where it has 

 assumed a reddish tint, the peroxide of iron is 

 responsible for the discolouration. Where of a 

 chocolate tint, the peroxide of manganese, also a 

 derivative of the decomposition of volcanic material, 

 has coloured the clay. 



It is believed that at these great depths the 

 materials have been gradually accumulating fir ages. 

 Here upon the clays were found enormous quantities 

 of sharks' teeth, ear-bones of whales, and other frag- 

 ments of bones. Not that such areas formed places 

 of sepulture for marine animals, more than any other 

 parts of the ocean floor, but it was owing to the slow 

 accumulation of the matrix that the thousands of 

 generations of creatures which had lived and died, 

 had left their remains sciU uncovered in this extremely 

 slow-growing deposit. In the more rapidly, but still 

 very slowly forming oozes, such remains become 

 buried sooner. Sir John Murray has said that in 

 some of these red clay areas there is a probability 

 that but a few inches only have been deposited since 

 Tertiary times, and that the observations made tend 

 to support the view of the vast antiquity of the great 

 ocean basins. It is estimated that no less than 

 50,000,000 square miles of ocean floor are covered by 

 these clays. 



In these deep sea areas, many of the remains have 

 been coated, to a varying extent, with manganese, 

 itself a tremendously slow process. The presence of 

 this mineral and the manner in which it has incrusted 

 and preserved animal remains, reminds the geologist 

 very forcibly of the similar part taken in our Chalk by 

 flint, since this occurs, like the manganese, both in 

 nodular form and as incrustations. 



The proportion in which certain sphetides of native 

 iron are found on the sea-bottom, serves as a measure 

 of the rate of deposition of the pumice-stone clays. 

 These spherules are derived from interstellar space, 

 and are known as "cosmic dust." They may fall 

 anywhere or everywhere, but it is only where not 

 masked by detritus from a neighbouring coast that 

 they are met with in sufficient quantities to become 

 appreciable. 



With the exception of the deep-sea clays, the most 

 abundant ocean-floor covering was found to be Globige- 

 rina Ooze. As a matter of fact there were but few 

 places, where soundings were made, that did not con- 

 tain some species of Globigerina, but it was only where 

 they occurred in very great numbers that the deposit 

 was characterised by the name. It was remarked that 

 this ooze most nearly resembled Chalk when it occurred 

 in the more shallow seas, down to a depth of about 

 1,000 fathoms. Below this depth the ooze assumed a 

 rose-tinted aspect, passing at other times into a red or 

 a dark brown, due respectively to the presence of the 

 peroxide of iron and the peroxide of manganese. 



In enclosed seas Globigerina Ooze was remarkably 

 absent, but where found it was at a depth varying 

 from 250 to 2,900 fathoms. It is noticeable that at 

 3,000 fathoms scarcely a foraminifer or a calcareous 

 organism remained. The Globigerina Ooze is esti- 

 mated by Murray and Irvine to occupy no less than 

 47,752,500 square miles of the oceao floor, the mean 

 proportion of carbonate of lime being 64'53 per cent. 

 One of the discoveries most interesting to geologists 

 was that the various forms of life met with in the deep 

 seas were wonderfully uniform, whether they came 

 from the North or South Atlantic, the great Southern 

 Sea, or the middle of the Pacific. In all geological 

 formations older than the Chalk, to all intents and 

 purposes the same fossil fauna holds good in the same 

 fonnation, wherever it occurs. Possibly this simi- 

 larity is merely due to the fact that such geological 

 formations are really deep-sea deposits. Ordinary 

 estuarine and fresh-water deposits, where they are 

 clearly recognisable as such, would be, of course, 

 removed from this generalisation. 



The Atlantic or Globigerina Ooze is thus described 

 by Sir Wyville Thomson in his work, "The Depths 

 of the Sea" : — "The upper layer is soft and creamy 

 in consistence and of a yellowish colour, while the 

 main mass is firmer and more tenacious, of a grey or 

 bluish-grey tint. Under the microscope the surface- 

 layer was found to consist chiefly of entire shells of 

 the foraminifer, Globigerina bulloidcs, large and small, 

 and fragments of such shells mixed with a quantity of 

 amorphous, calcareous matter in fine particles, a little 

 fine sand, and many spicules, portions of spicules and 

 shells of Radiolaria, a few spicules of sponges and a 



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