TWENHOFEL: EXPEDITION TO THE BALTIC PROVINCES. 345 
other exposures present similar features, and the fact of such variation 
is altogether too evident to be 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 were 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 odlitic 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 
odlitic zone of southern Gotland, while Hedstrém 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 Lindstrém 
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,’ but 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. Itis 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 débris, 
undoubtedly blown out to sea by the prevailing tradewinds. We 
1 Guide book 11th. internat. geol. congr., 1910, no. 19, p. 8, quoted by Munthe. 
2Hedstrém. 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. 
