114 Evolution and Distribution of Fishes 



discontinuity, and so suggest volcanic action and displace- 

 ment. As for the Upper Silurian, so also for the Devonian, 

 we hope to show that two very distinct assemblages of 

 organisms characterize freshwater and marine deposits 

 respectively. 



Over wide areas of the North Eastern United States, 

 of South England, of Northern France, Central Germany 

 and Russia, South-west China and New South Wales thick 

 deposits of marine strata, that enclose typical marine fos- 

 sils, have been followed out. To these the term Devonian 

 was restricted by some geologists. But distinct from such 

 beds, and often forming enormous stratified masses, occur 

 other deposits with a very different biological assemblage, 

 and which have had a very different physical history from 

 the former. To these the older geologists gave the name 

 Old Red Sandstone. In endeavoring to unravel gradually 

 the history of the relation and succession of these two 

 groups, the writer was at first completely bewildered, then 

 puzzled, still later in hopeful mood, and finally satisfied 

 as to exact explanations. Such mental phases were due 

 to the frequent description by the earlier geologists, of 

 blocks of fossils which had been collected in some one, or 

 in a few neighboring regions, but which had not been 

 exactly labeled as to stratigraphic occurrence and succession. 

 Or again geologists in the field had accumulated collections 

 which were described by colleagues, who had never or 

 seldom visited the rocks. Not unfrequently also one could 

 trace a distinct prepossession or mental bias, on the part 

 of some writer for an already-formed theory or opinion. 



In study of these we may again start with European 

 strata, as they have been longer and more fully compared, 

 than those of North America or other continents. As now 

 synopsized by Geikie (7:982-999), by Prestwich (66:11: 

 74-82), by de Lapparent ((57:11:836), and by Roemer- 

 Frech (<5<^: 1 : 34-55 ; II : 117-256) the rocks of central Eu- 

 rope from south England eastward, and to which the term 

 Devonian was first applied by Murchison and Sedgwick, 

 are clearly marine, but largely littoral-marine in origin. 

 The nature of the rocks, the abundant lists of marine fos- 



