94 NOTES ON CORAL REEFS, ETC. 



Among the silica-secreting denizens of the surface waters 

 are simple animal forms such as the liadiolaria, and vegetable 

 organisms belonging to the Diatomaceoe. Among those that 

 secrete carbonate of lime are the Pteropoda, and familiar 

 representatives of this class are Hyalea nnd Gleodora, specimens 

 of which, with their beautiful translucent shells, are often 

 collected by means of t'^w nets let dowu from passenger 

 steamers or sailing ships. Of other forms, the Foramiiiifera 

 comprise most of the minute animals with dense shells, such 

 as those of the Glohigerince and Orhulince. 



The thin si) ells of the Pteropods, after the death of their 

 tenants, sink so slowly that they are dissolved away before 

 reaching great depths, and their remains are rarely found 

 where the depth exceeds lOOO fathoms. The dense shells of 

 the Glohigerince sink to much greater depths before being 

 dissolved, and are found all over the sea bottom down to 

 about 2,500 fathoms. The remains of the surface organisms 

 reaching greater depths are chiefly the silicious skeletons of 

 Ecidiolarians and Diatoms. These are blended with the other 

 materials slowly accumulating in the deepest parts of every 

 ocean, forming a reddish brown sediment which has received 

 the distinctive name of Ued Clay. 



The main point which has to be grasped, and it is not easy 

 to realise it, is that, during past ages and up to the present 

 day, there has been a never-ceasing downfall of the remains 

 of these short-lived organisms, which has gradually raised 

 the sea bottom over wide areas to a considerable altitude. 

 The deposit thus formed is now known under the general 

 name of ooze. That portion of it which is found at a depth 

 not exceeding 1,000 fathoms has been called Pteropod ooze, 

 from the large percentage of the remains of Pteropods con- 

 tained in it. The same kind of formation extending from 

 1,000 to 2,500 fathoms is called Glohigerina ooze for a similar 

 reason, and this is succeeded by Diatom and Radiolarian 

 ooze until these last are blended with the ultimate residuum — 

 Ued Olay—ou the floor of the deepest seas. In connection 

 with this part: of the subject it may be noted that the great 

 chalk formation, which occupies a large extent of country in 

 the south of England, and on the continent of Europe, and 

 which beneath London is about 700ft. thick, is practically 

 identical with the deposits that have just been mentioned. 

 It has been proved by microscopic examination that chalk is 

 nothing more than what may be described in general terms 

 as Glohigerina ooze in a consolidated form, which had accu- 

 mulated by slow oceanic sedimentation, long ages ago, ou an 

 ancient sea floor. 



The conclusion arrived at by Dr. Murray of the Challenger 

 Expedition was, that coral reefs and islands were gradually 



