442 



PRINCIPLES OF STRATIGRAPHY 



cent discoveries of the wonderfully preserved Cambric fauna of 

 British Columbia, had not its equal the world over. 



Conditions like those which existed in the Solnhofen region dur- 

 ing Jurassic time are unknown in any modern coral reef ; neither 

 were they repeated in any of the numerous reefs known from the 

 Palaeozoic, Mesozoic or Cenozoic periods of the earth's history. 

 They remain an example of a unique type of reef resulting through 

 a remarkable concatenation of topographic, climatic, lithologic and 

 faunistic conditions not met before, nor since attained. 



The Sponge Reefs of the Swabian Jura. In the Upper or 

 White Jura of Swabia, massive, structureless limestones and dolo- 

 mites are found, which prove to be reefs built largely of lime-secret- 



FiG. 97. "Sponge Reef" of the lower White Jura of Swabia. (E. Fraas.) 

 R — Structureless reef limestone, a, /3, 7, associated bedded Juras- 

 sic limestones, marls, etc. S. Talus. (From Kayser.) 



ing sponges. These reefs from their massive character stand out in 

 reUef through the erosion of the softer enclosing rock, and so form 

 pinnacles, buttresses or even isolated outliers of the Swabian Alb, 

 which since the earliest days have formed the favorite site of castles 

 and other strongholds. These sponge reefs sometimes extend 

 through several subdivisions of the Upper (White) Jura, as shown 

 in the accompanying figure (Fig. 97), after E. Fraas, where the 

 reef is surrounded by bedded sediments of the stages a, P and y- 

 The reef contains many other well-preserved organisms besides 

 sponges, among which latter the genus Cnemidiastrum predomi- 

 nates. On the flanks of the reef a breccia of reef rock is found 

 with some clay, and this passes into the normal bedded strata of 

 clastic limestones. 



MiocENic Reefs of the Austro-Russian Border. The low 



