CONODONTS IN ARENIG-LLANDEILO FORMATIONS. 239 



into small clusters by mutual attraction, which may be of 

 volcanic and cosmical origin. In some of them there are also 

 perfect rhombs of a mineral which at first I took to be scales 

 of some kind of animal, as they often show on the surfaces 

 of the prepared shales as minute lozenge-shaped bodies. The 

 largest measure about "2 of a mm. in their greatest length, 

 and some are exceedingly minute. They have evidently been 

 developed in the shales since their deposition, and are probably 

 crystals of calcite. 



The great bulk of the material making up the cher1>shales is 

 composed of exceedingly minute particles of dust, and the ocean 

 in which the cherts and their intercalated shales were deposited 

 was evidently never discoloured by sediment unless by particles 

 as minute and scarce as those which give the fine " sea-green " 

 to the purest parts of the oceans of the present day. The shales 

 merge into the chert-bands, and the latter, besides being crowded 

 with Radiolarians, often contain a lot of dust, rarely small cubes 

 of pyrite, more frequently minute rhombs of calcite, and are 

 sometimes stained by carbon, manganese, and iron salts. 



I made a pretty extensive microscopical examination of the 

 chert>bands, to see if I could find Conodonts, but was successful 

 in obtaining only two specimens. The Conodontians evidently 

 did not agree with the Eadiolarians, and had migrated to other 

 parts of the ocean during the intervals of time when the latter 

 held supreme sway. 



From a study of the chert Radiolarians, Dr. Jennings Hinde 

 thinks that they lived in water as deep as those of the present 

 day do, viz., in an ocean over 12,000 feet in depth.* 



Since Silurian times the only Radiolarian beds known to 

 geologists are those in the Devonian Rocks of New South 

 Wales;! the Culm Strata of Devonshire, now supposed to be 

 the deep ocean equivalents of the Mountain Limestone of 

 England;; Mullen Island, ofiE the western coast of the Lizard, 

 age unknown ; § Port Darwin, Australia, age unknown ; || Bar- 



* Annals and Mag. Nat. Hist., July, 1890. 

 + Quar. Jour. Geol. Soc, February, 1899. 

 t Quar. Jour. Geol. Soc, 1895. 

 § Quar. Jour. Geol. Soc, May, 1893. 

 II Qunr. Jour. Geol. Soc, May, 1893. 



