422 PRINCIPLES OF STRATIGRAPHY 



The Gotland reefs often rest on marly clay beds or on oolitic 

 sands. When the latter is the case, it appears that these oolitic 

 sands immediately succeed a shallow-water, if not terrestrial quartz 

 sandstone, and are themselves of very shallow-water origin, show- 

 ing an abundance of wave activities. The reefs were thus evidently 

 built in water of no considerable depth, as is further shown by the 

 structure of the reef borders themselves. Where the reef rests on 

 the marl shales, the latter show evidence of relative plasticity at 

 the time of reef formation, for the reefs, with growing size and 

 weight, sank into the marly muds to some extent, forcing the 

 mud away in either direction or causing the layers to bend down 

 under the reef. 



The reefs of this section apparently formed a barrier parallel 

 to the old Scandinavian land, in part of which graptolite-bearing 

 shales seem to have been deposited at the same time. The earlier 

 reef series shows its proximity to the land by the fact that, in the 

 lagoons between the reef mounds, terrestrial or semiterrestrial 

 organisms, the eurypterid Pterygotus and the scorpion Palsophonus, 

 were found together with marine organisms. It is highly probable 

 that the reef was doJ;ted with islets, and that perhaps even fresh- 

 water lagoons existed, and this would serve to explain the exist- 

 ence of these continental organisms. It is noteworthy that the 

 Pterygotus beds occur at the lower boundary of a distinct hiatus, 

 which marks a period of land condition for the entire island, and 

 was probably of much greater extent. 



Upper Siluric Reefs of Nortli America. Reef structures are 

 known from the Upper Siluric (Monroan) formation of western 

 Ontario and ]\Iichigan and the equivalent Lewistown limestone of 

 Pennsylvania. They probably occur in formations of similar age 

 in northwest Canada. The best example of these reefs occurs in the 

 Anderdon limestone quarry, about a mile northeast of Amherst- 

 burg, Ontario. (Sherzer and Grabau-8o: ^j.) Its thickness is 

 only from 6 to 8 feet, but its horizontal extent is unknown. It is 

 probably of the nature of a series of lenses of organic limestone, 

 surrounded by and passing into the finest lime mud strata (cal- 

 cilutytes) derived from the destruction of the reef. The organisms 

 of the reef are mainly stromatoporoids, Stromatopora galtense and 

 Clathrodictyon ostiolatnin predominating. With these occurs the 

 small branching stromatoporoid. Id'wstroma nattressi, which was es- 

 pecially adapted to capture the coral sand and mud produced by the 

 grinding action of rolled Stromatopora heads. Corals of the genera 

 Favosites, Diplophyllum and Cladopora, all well adapted to capture 

 coral sand, on account of their loosely branching habit, and simple 



