RADIOLARITES 459 



Radiolaria have been found in cherts of various ages from the 

 Ordovicic on, and some of these cherts are beHeved to be due to 

 the segregation of siHca furnished by the Radiolaria and other silica- 

 secreting organisms. 



The Jurassic Radiolaritc of the Alps. In certain portions of the 

 Austrian Alps, a remarkable deposit of radiolarian mud is found 

 in the upper Jura. (Hahn-43, i:j6'p.) This radiolarite, as it is 

 called, consists of intensely blood-red, more rarely greenish, jaspery 

 layers, alternating with dense, brown-red or greenish gray quartz- 

 ose and argillaceous marls, the total having a thickness varying from 

 10 to 25 m., the increase being due to increase in the marly layers. 

 Under the microscope it appears as an extremely fine-grained, al- 

 most homogeneous mud rock, filled with minute spherical crystal 

 bodies, referable to Radiolaria. The original ornamentation of these 

 bodies is seldom preserved in the argillaceous layers, but the jasper 

 contains Spumellarians in a wonderful state of preservation. This 

 rock thus appears to be of the type of radiolarites which represent 

 the deep-sea radiolarian oozes, and are found to-day in depths of 

 from 3,000 to 7,000 meters. 



These radiolarites rest on Upper Lias, or on variegated am- 

 monite limestone or breccias of Middle Liassic age. Some indica- 

 tions point to a disconformable relation with the underlying rock, 

 there being apparently an absence of Dogger and part of the Malm. 

 This would suggest land conditions with erosion prior to sub- 

 mergence, the sinking being a rapid one down to the depths at 

 which such ooze will accumulate. 



Upward the radiolarite passes into gray calcilutytes with 

 Aptychus of Tithonic age. This indicates a gradual shoaling of the 

 water, which culminated in land conditions in the next succeeding 

 period. 



Similar deposits of Radiolarite have been described from the 

 Lower Carbonic beds of the Rhine district (Wilckens-92 : J5^) and 

 from various parts of Great Britain. These examples of older 

 radiolarites are believed to be of shallow-water origin. Such cherts 

 are well shown in the Carboniferous (Mississippic) limestone series 

 of Gower in western England ( Dixon and Vaughan-25 : 5^"/). They 

 are finely and sharply laminated, many of the lamin?e being lenticu- 

 lar or wedge-shaped. They are also found in the lower Culm of 

 Devon (Codden Hill beds), from which they were first described 

 by Hinde and Fox, who, however, regarded them as deep-sea de- 

 posits. From the nature of the de])osits. themselves, as well as from 

 the character of the including rocks, others have come to the con- 



