346 bulletin: museum of comparative zoology. 



frequently found floating on the surface masses of vegetation, more or 

 less water-logged, and ready to sink. The contents of some of our 

 trawls would certainly have puzzled a palaeontologist; between the 

 deep-water forms of Crustacea, annelids, fishes, echinoderms, sponges, 

 etc., and the mango and orange leaves mingled with the branches of 

 bamboo, nutmegs, and land shells, both animal and vegetable forms 

 being in great profusion, he would have found it difficult to decide 

 whether he had to deal with a marine or a land fauna." ^ 



It is not improbable that similar conditions obtained in Silurian 

 days. Eurypterids occur at about the same level, and their presence 

 is susceptible of the same explanation, even if they were not them- 

 selves marine. 



In the south of Gotland I do not consider that any evidence of an 

 unconformity indicating a land interval is present nor was any seen in 

 the waterfall section at Wisby, but the observations made at that 

 section were not extensive, nor detailed. Hedstrdm described a dis- 

 conformity between the Lower and Upper Gotlandian in the precipice 

 south of Gustaf svik, indicated by a " thin marly and gravelly water- 

 worn layer, containing, amongst other things, worn Gastropods of the 

 genera Trochus, Pleurotomaria, Horistomia etc.," and he has little 

 doubt of the actual occurrence of erosion, but he does not state whether 

 he considers this as indicative of a land interval.^ I have not seen the 

 locality and so can not give further data, but the presence of pebbles 

 and worn shells may be equally well explained by contemporaneous 

 erosion in a coral reef channel. It appears that the question of land 

 in Middle Silurian time, within the present area of Gotland, remains 

 to be proven, and the idea is favored that marine conditions prevailed 

 over the present limits of Gotland throughout the time of deposition 

 embraced between the youngest and oldest deposits. 



Evidence of shallow water. That the Gotland rocks are the deposits 

 of very shallow water is evidenced by the extraordinary development 

 of coral reefs, the strongly developed edaphic modification shown by 

 the fossil faunas, and the extensive lateral gradation of sediments. 

 These show that the sea bottom was sufficiently shallow to closely 

 respond to land conditions, and that there were numerous local bar- 

 riers so near the surface as to produce a great variety of differing con- 

 ditions. Among these local barriers the most important were the 

 coral reefs which protected colonies within them and hindered migra- 

 tion over, or around them. 



Agassiz. Three cruises of the Blake, 1888, 1, p. 291. 



Hedstrom. Guide book 11th. internat. geol. congr., 1910, no. 20, p. 23, fig. 4. 



