420 PRINCIPLES OF STRATIGRAPHY 



stromatoporoids. with perhaps a moderate development of Favo- 

 sites and other corals. On the flanks of the reef numerous smaller 

 corals are found, together with an abundance of crinoids, brachio- 

 pods, molluscs, and some trilobites. The peripheral dips of the 

 bedded strata are of the usual character, and here, as in the other 

 reefs, a repeated interfingering of the reef rock and the bordering 

 clastic strata is marked. Several miles from Groth's quarry, just 

 described, is Anschiitz quarry, on the road to Grafton. Here the 

 bedded strata on the flanks of the reef are well shown, their char- 

 acter being that of a typical calcarenyte locally known as sand- 

 stone. The general arrangement of the Siluric reef mounds of 

 Wisconsin is in more or less linear series parallel to the old shore 

 of the period, but probably at a considerable distance from that 

 shore. (Chamberlin-15.) The arrangement appears to have been 

 that of a barrier reef, the eastward extent of which can be traced 

 through northern Alichigan, where an abundant coral fauna, prob- 

 ably in reef association, is found on Drummond Island and else- 

 where. Its eastern extent appears to have been in the vicinity of 

 Lockport, New York, but neither there nor in Michigan have actual 

 reef structures been observed. Farther south occurred independent 

 reefs, remains of which are now found at Louisville, Kentucky, at 

 the Falls of Ohio. Probably many other reefs of this age exist in 

 the Hudson Bay region, where strata of this age still await detailed 

 exploration. 



Siluric Reefs of Gotland. One of the finest exhibitions of Palae- 

 ozoic coral reefs is found on the western coast of the Island of 

 Gotland in the Baltic Sea. (Wiman-9.3; Hedstrom-47 ; Munthe- 

 65.) Here the erosion by the waves both at the present altitude 

 and during a former period of higher water has exposed a series of 

 wonderful sections through the ancient reefs such as are probably 

 not to be found anywhere else in the world, though equally fine 

 sections of Mesozoic and later reefs are well known. There appear 

 to be two main horizons in which the reefs occur, separated by a 

 disconformity, one series being best exposed in the northern and 

 the other in the southern part of the island. 



In general aspect the "reefs" are lenticular mounds locally 

 known as "klintar." They stand out as bold cliffs on the shore, 

 their massive, structureless character offering great resistance to 

 both wave and atmospheric erosion, whereas the bedded enclosing 

 strata are more rai)idly reduced by both agencies, and so produce 

 indentations and reentrants in the coast line. (Fig. 85.) 



Unlike the Siluric reefs of Wisconsin, those of Gotland are 

 practically unaltered by subsequent dolomitization, and hence their 



