twenhofel: expedition to the Baltic provinces. 347 



The sandstone layers in the south of Gotland are extremely sug- 

 gestive and interesting. They appear to grade northward into shales, 

 suggesting that the source of the materials was southward. The 

 sands are the deposits of quite shallow water, and are probably not 

 wind and wave deposits of a shore, the absence of decided crossbedding 

 rather strongly arguing against the latter vieAV. They contain fossils 

 only near the base and near the top and are overlain by oolites. The 

 cleanness of the sands and the absence of crossbedding and fossils 

 suggests the drift sand zone of Godwin-Austin, a deposit of fairly 

 ciuiet water; while the overlying oolites with fossils could well have 

 ■accumulated in the shallow water after either some extra-Gotland 

 territorial ph^'sical changes or changes in the development of the 

 coral reefs, modifying the direction of the currents, had caused the 

 sand to cease to drift. A most important fact connected with the 

 sandstone beds is that in the sea over southern Gotland they elimi- 

 nated the corals during the duration of the deposition of the sands, 

 and hence no coral reefs appear to pass through them. 



The coral reefs. Many students have commented on the extensive 

 development of the reefs of Gotland and they are well worthy of com- 

 ment. They are present in practically every parish of the island, 

 and the surface and shore topography of Gotland have been largely 

 controlled by the fact of their occurrence, and these in turn to a large 

 extent have influenced the past and the present activities of the 

 Gotlanders. The organisms which played the greatest part in the 

 development of the reefs belong to the Stromatoporoidea, and there 

 are probably a half score of these for every colony of all the others 

 put together. Crinoids also made great contributions to the material 

 of the reefs. Some of the reefs are of large size. At Hoburgen klint 

 one of them is fully one hundred and fifty yards wide and reaches from 

 the oolite to the top of the cliff, a thickness of about seventy-five feet. 

 The corals of the reefs are frequently little changed from their original 

 condition, and in pockets between the coral growths are small masses 

 of clay and sand which are remarkable for the excellent fossils which 

 they contain. In general, the coral masses offer greater resistance 

 to erosion than do the surrounding or underlying rocks, so that in the 

 clift's they project as salients or overhanging masses and in the fields 

 as knolls, on the northward or stoss side of which, the side from which 

 the ice came, the surrounding rock has all, or nearly all, been eroded 

 away, while on the side opposite the direction of ice movement the 

 protection of the coral masses has preserved to some extent the sur- 

 rounding rock, which is generally a crystalline limestone. These 



