406 



HISTORICAL GEOLOGY 



FIG. 429. Probable 

 appearance and 

 structure of a com- 

 mon Jurassic mol- 

 lusk (Belemnites) 

 allied to the mod- 

 ern cuttlefish. The 

 shell is the shaded 

 portion below and 

 is the only part 

 usually preserved. 

 (After Pictet.) 



FIG. 431. A Juras- 

 sic oyster shell 

 (Ostrea). 



United States. In the West only barren 

 sands and clays, such as accumulate on 

 land, and probably under conditions of dry 

 climate, seem to have been deposited in 

 the earlier part of the period. Later a 

 broad tract extending from Alaska south to 

 Wyoming and Utah subsided enough to let 

 in an arm of the sea. That this sea was 

 shallow is indicated by the character of 

 the sediments, which are shales and sand- 

 stones, with occasional beds of limestone. 

 The fossils in the limestones are not only 

 the remains of animals which lived in the 

 ocean, thus proving that this was the water 

 of the sea and 

 not of a large 

 lake, but they 

 were most near- 

 ly related to the 

 animals which 

 lived at the 

 same time on 



the COast of FlG 430. A large and oddly 

 Alaska, and ornamented pelecypod (Trigo- 

 CVL i Dia) f the Jurassic period. 



even in Siberia. 1 



Their affinities with the Californian types 

 are much less close. From this we may 

 infer that the sea came in from the far 

 north rather than from the west (Fig. 432). 

 This inundation of the Northwest during 

 the late Jurassic was of short duration only. 

 At the close of the period, changes of level, 



1 At this time Siberia and Russia were largely sub- 

 merged by an expansion of the Arctic Ocean, and 

 broad bays spread southward from this into central 

 and western Europe joining the ancestral Mediter- 

 ranean Sea, which was much larger then than DOW. 



