twenhofel: expedition to the Baltic provinces. 345 



other exposures present similar features, and the fact of such variation 

 is altogether too evident to he opposed. It appears very doubtful if 

 any single division is continuous without much variation over the 

 entire island. This, and other features to be noted later, have made 

 the stratigraphic determinations and correlations extremely difficult. 

 Since changes in sediments are invariably accompanied by faunal 

 changes the problem is one of great complexity and much difficulty. 



Unconformities in the Gotland section. Discordances of strata are 

 not uncommon in the Gotland section; but where seen by the writer 

 they have no great significance and bear no other interpretation than 

 that of contemporaneous erosion and refilling, that is, they w^ere made 

 by erosion of the sea-bottom during the times of deposition of the 

 sediments, or they have been produced by the overgrowth of corals on 

 sediments or the covering of reef growths by sediments. Some of the 

 conglomerates which have been mentioned by various writers have 

 been considered as evidence of a transgressing sea. Such conglomer- 

 ates are present in great number, but they always occur as lenses and 

 are always local. Many of the rocks which have been called con- 

 glomerates are certainly not such, since the "pebbles" have a con- 

 centric structure and are either of oolitic or organic origin. At any 

 rate, these conglomerates bear no relation whatsoever to a transgress- 

 ing sea. Holm ^ considers the probable existence of an unconformity 

 at the top of the lower division whose summit is placed above the 

 oolitic zone of southern Gotland, while Hedstrom mentions a dis- 

 cordance which is situated at about the same position, that is, between 

 the Lower and Upper Gotlandian beds - and at about this same level 

 the Silurian scorpion, Palaephonus nuncius Thorell and Lindstrom 

 was discovered in association with marine forms. Holm thought that 

 the presence of this land form bore on the question of discordance and 

 the probability of a Middle Silurian land interval on Gotland,^ bat to 

 the writer it fails to bear that, as the only, or even the probable inter- 

 pretation. It might have attained the bottom of a shallow sea in 

 many ways. It is by no means rare today to see land insects and other 

 land organisms floating in the sea, and A. Agassiz states that " It was 

 not an uncommon thing to find at a depth of over one thousand 

 fathoms, ten or fifteen miles from land, masses of leaves, pieces of 

 bamboo and of sugarcane, dead land shells, and other land debris, 

 undoubtedly blown out to sea by the prevailing tradewinds. We 



1 Guide book 11th. internal, geol. congr., 1910, no. 19. p. 8, quoted by Munthe. 



2 Hedstrom. Guide book 11th. internat. geol. congr., 1910, no. 20, p. 9. 



3 Guide book 11th. internat. geol. congr., 1910, no. 19, p. 8, quoted by Munthe. 



