438 



PRINCIPLES OF STRATIGRAPHY 



bivalve molluscs, among which large and coarse types, such as oys- 

 ters, Pecten, Hinnites, Lima and especially the curiously twisted 

 Disceras predominate. The last is so abundant in some places, e. g., 

 Kelheim, that the beds have become known as the "Disceras lime- 

 stones." These molluscs flourished on the margin of the reefs, 

 where the wave activities were strong, and food plentiful. Large 

 and coarse gastropods were also plentiful, including Xerinea, Ceri- 

 thium, Turbo, Pleurotomaria and others. With these also occur 

 large brachiopods (Terebratula and Waldheimia), which occupied 

 the cavities in the reefs, and sea urchins, which had heavy skeletal 

 parts and crawled about on the reefs. All these are now incor- 

 porated in the coarse reef rock which constitutes a large portion of 

 this formation. These organisms are best preserved on the margins 

 of the reefs, the main portion having more commonly the appear- 

 ances of a structureless limestone or dolomite (Franken dolomite, 



Fig. 93. Section through the reef region (Jurassic) of the Altmiihl, in the 

 Pappenheim-Eichstadt region (near Solnhofen) in Bavaria. The 

 reef rock is structureless, the thin bedded "Plattenkalke" or litho- 

 graphic beds are shown in the lagoon-like depressions in the reef. 

 (After Walther.) 



Kelheim limestone). This rock is well shown in the gorge of the 

 Danube at Kelheim, and in numerous other natural and artificial ex- 

 posures in Franken. As a rule, the reef masses stand out in relief 

 in the present topography, while the inter-reef portion, occupied by 

 the thin-bedded calcilutytes (Plattenkalke), is often worn out into 

 a hollow or depression. The individual knolls seem to have had an 

 arrangement suggestive of atolls, though on the whole this series of 

 reefs is more properly to be regarded as forming a barrier reef to 

 the old Jurassic land. (Fig. 93.) 



The reef-rocks of the Swabian Alb have recently been studied by 

 Dr. Fritz Berckhemer,* who finds that the Hydrozoan Ellipsactinia 

 is one of the principal reef-builders, holding the place of the stro- 

 matoporoids of the Palaeozoic, and of the milleporoids of later time. 

 These, together with nullipores and the lime-sand derived from 



* Fritz Berckhemer. Eine vorlaufige Mitteilung iiber den Auibau des 

 Weissen Jura e (Quenstedt) in Schwaben. Jahreshefte d. Vereins f. Vaterl. 

 Naturkunde in Wurteniberg, 69 Jahrgang, 1913 pp. Ixxvi-lxxii. 



