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 view. They contain fossils 
only near the base and near the top and are overlain by odlites. The 
cleanness of the sands and the. absence of crossbedding and fossils 
suggests the drift sand zone of Godwin-Austin, a deposit of fairly 
quiet water; while the overlying odlites with fossils could well have 
accumulated in the shallow water after either some extra-Gotland 
territorial physical 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 odlite 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 
cliffs 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 
