440 



PRINCIPLES OF STRATIGRAPHY 



that the clayey beds are due to deposition of terrigenous dust, and 

 in them the terrestrial insect fauna was buried. 



It appears, then, that these lagoon-Uke depressions or basins in 

 the coral reefs of the Jurassic sea of that region were slowly filled 

 by the fine lime mud derived from the destruction of the reefs, by 

 terrestrial dust brought by the strong winds from the distant land 

 and by chemical precipitation of lime. Thus were formed the .fine- 

 grained lime deposits, which reach in places a thickness approaching 

 a hundred feet. In the more clayey beds and between the layers 

 were preserved the insects and plants blown from the mainland or 

 the marine types brought there during the flooding of the lagoons. 

 Walther has shown from the position of these remains that with 



Fig. 95. Section of the reef rock (Franken dolomite, Jurassic) of Kelheim, 

 Bavaria. The thin bedded "Plattenkalke" are shown by horizontal 

 lining. Those in the upper left-hand portion show distortion 

 through gliding. ("Krumme Laage," see page 781.) (After 

 Walther.) 



few exceptions they were brought there dead, and left as stranded 

 carcasses on the ooze of the lagoon bottom, which was mostly cov- 

 ered by little water if not altogether exposed. The repeated evi- 

 dence of shallow water, and even complete exposure of the lagoons 

 to the air, suggests that these deposits were accumulating during a 

 slow subsidence of the region and that the filling of the lagoons in 

 general kept pace with the sinking of the reefs. The absolute uni- 

 formity of the entire series of thin-bedded limestones further shows 

 that the physical conditions remained uniform during their deposi- 

 tion. Walther assumes that the old shore of the Vindelician land 

 was some 20 km. to the south, but no clastic mud was carried by the 

 currents to these reefs. Instead, the terrigenous dust incorporated in 

 the thin sediments represents wind-blown material. It is in these 

 dust-bearing layers that most of the fossils are found, especially the 

 insects, of which there are 72 genera and 103 species, 35 per cent. 



