TWENHOFEL: EXPEDITION TO THE BALTIC PROVINCES. 343 



IVb. Ostracodan limestone, or marl shales and crinoldal limestone. 

 IVa. Bottom stratum with Stromatoporan limestone and Spongio- 



stroma. 

 III. Upper cliff level of varying composition in different localities. 

 11. Lower cliff level. 

 I. Stricklandinia marl.^ 



Below the lowest visible strata of the northern end of the island are 

 others which have been revealed by boulders on the shore near Visby. 

 These consist of red shales containing the coral Arachnophylkim, on 

 account of which they have been called the Arachnophyllum shales. 



Each of the above classifications is subject to the same criticism, 

 in that divisions have been named after fossils which by no means 

 always occur, or after some lithic characteristic which is only locally 

 present. 



Attitude of the strata. This is the bone of contention over which 

 opinion has clashed. In many places there appears to be a definite 

 dip southeastward, but in an extremely great number of other places 

 it pan as definitely be determined that a pronounced dip in the oppo- 

 site direction obtains. Since, however, the lowest visible division, or 

 zone, is exposed only at the northern end of the western shore, near 

 Visby, it appears reasonable to assume that there is a general gentle 

 southeastward dip, from which, however, there are many reversals 

 of inclination. The island of Storo Karlso excellently illustrates the 

 directional variations of inclination. On the western side of the 

 island the Marl shales are beautifully exposed to the south and north 

 of the point on which the light-house stands and the beds dip into the 

 sea. On the eastern side there are no exposures of the Marl shales, 

 but overlying beds are extensively exposed and dip eastward into the 

 sea, while on the coast of the main island directly opposite (Gotland), 

 the ]Marl shales again appear with southeastward inclination (Plate 4). 

 At the Lau Canal the beds dip northwest at an angle of about fifteen 

 degrees. These facts bear but one interpretation, i. e., very little 

 reliance can be placed on local inclinations in reaching conclusions 

 relating to the general attitude, and from inclinations alone one can 

 reach any conclusion desired, merely by selecting the proper place to 

 reach the conclusion. 



Variation of sediments and faunas. There is nothing more striking 

 in the Gotland section than the horizontal variation of the sediments 

 and their contained faunas. These variations are to be seen in many 



' Hedstrom. Guide book 11th. internat. geol. congr., 1910, no. 20, p. S. 



