TYPICAL GKAPTOLITE SPIALES 961 



them than about any other graptolite horizons in the world, and also be- 

 cause the senior author has made a special study of these regions in the 

 field in Sweden under the guidance of Doctors Moberg and Wiman and 

 in Scotland under the guidance of Doctor Benjamin Peach. We are not 

 ready at present to say that the conclusions which we draw from the 

 Lower Siluric of Sweden and south Scotland are necessarily applicable to 

 all of the graptolite shales; but we shall be satisfied if we can offer con- 

 vincing evidence that tlie two shale series which have been most unques- 

 tionably considered as deep-water deposits are really of near-shore origin. 

 Before proceeding any further, it will be necessary clearly to define grap- 

 tolite shales or GraptoUtenschiefer. As currently understood, these com- 

 prise formations of variable thickness, in which bands of black carbo- 

 naceous shales, generally of slight thickness, alternate with gray shales and 

 more or less frequently with arenaceous beds. The black shales usually 

 contain the graptolites, sometimes in vast numbers, and with them rarely 

 occur other marine organisms, chiefly inarticulate brachiopods; the gray 

 shales, as a rule, are barren of fossils, though a scattered representation 

 may occur. Formations rich in other marine organisms and containing 

 in addition either sporadic graptolites or interbedded layers, with a con- 

 centration of the remains of these organisms, are especially excluded from 

 our' present discussion. 



The Svstedish Region 



The areas to be looked at in detail provide us with two distinct lines of 

 evidence. In Sweden, in the Lower Siluric, there was a steady advance 

 of the sea northward over a land surface which had previously been sub- 

 jected to erosion. During the advance there was a continuous overlap of 

 the black shales on the eroded surface of the Ordovicic formations, the 

 black shale zones rising in the geological scale as one goes northv^ard. 

 Thus we have here a progressive overlap, accompanying a positive move- 

 ment of the strand-line, and the black shale facies advances with the ad- 

 vance of that line. In southern Scotland, on the other hand, we see the 

 strand-line being gradually pushed seaward, not by any diastrophic move- 

 ment, but by the encroachment of the continental deposits which fill up 

 the littoral areas of deposition and force a retreat of the sea. 



The Moffatdale Region 



The Lower Siluric in the Moffatdale region consists of 98 feet of black 

 and gray shales, the graptolites occurring in the thin black shale seams. 



