FOSSIL REEFS: JURASSIC 439 



the destruction of the reef, make up tlie "reef masses" which sur- 

 round the basins in which the platy Hmestones (Plattenkalke) were 

 deposited. In many cases, these Hydrozoa reefs rest upon or over 

 the sponge reefs of a lower horizon. 



The presence of islands on these reefs is shown by the eolian 

 more or less oolitic limestones with pronounced eolian cross-bed- 

 ding, shown in the quarries of Schnaitheim and Zandt. These have 

 precisely the structure of the eolian rocks of Bermuda. Walther 

 suggests that the organisms of the reefs near Nattheim were 

 abruptly killed and preserved by the mud spread over them, and so 

 they escaped the usual fate of the coral reef organisms, which are 

 destroyed by boring organisms, and changed to structureless dolo- 

 mite. Subsequent silicification has resulted in the w^onderful preser- 

 vation of this coral fauna, for which this region has become famous. 



The basins containing the fine and thin-bedded lithographic rock 

 form the strongest possible contrast to the enclosing reef mass. In 



Fig. 94. Diagrammatic section of a lagoon in reef-rock, with lithographic 

 limestone layers of extreme thinness (Plattenkalke) occupying the 

 depressions. (After Walther.) 



these sediments of impalpable lime mud the most delicate organisms 

 were preserved with a marvelous perfection. The feathers of the 

 ancient bird, Archseopteryx ; the wing membrane of the flying 

 saurians, the dragon-flies and other insects, with the veining of their 

 wings perfectly retained, are wonderfully well preserved, and even 

 the delicate jellyfish left its impressions in marvelous perfection. 

 Sometimes secondary reefs occur in the midst of the basin, indicat- 

 ing a temporary encroachn\ent of the reef builders, which later on 

 were again overwhelmed by the fine mud which produced the litho- 

 graphic calcilutytes. The strata are often very thin, and very uni- 

 formly bedded. The heavier bedded ones are used for lithographic 

 purposes, the thin ones for roofing and flagging purposes. At inter- 

 vals argillaceous layers occur, and some of the beds retain a mud- 

 crack structure. The evidence adduced by Walther goes to show 



