520 A. W. GRABAU rALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



young alike are swept seaward by the swollen rivers, which in their bot- 

 tom portions will roll along sands and pebbles, while the Eurypterid 

 exoskeletons and the fine mud are held more or less in suspension. If 

 the water enters a lagoon or arm of the sea, in which wave activity is 

 slight, these exoskeletons will sink to the bottom with the muds brought 

 in by the flood and will become buried in the mud layers. Over the delta 

 and the floodplains of the distributaries the muds will settle as the waters 

 subside, and with them the exoskeletons will sink to the bottom. Finally, 

 these muds become exposed to the air, are dried, crack and crumble, and 

 are blown about by the winds, and at a subsequent flooding they are re- 

 arranged by the waters and finally covered by coarser sediments. Dur- 

 ing this process the larger exoskeletons must of necessity suffer more or 

 less dismemberment, which the young ones readily escape by virtue of 

 their small size. The skeletons which were swept into the lagoons es- 

 caped this destruction because the general absence of waves prevented the 

 rearrangement of the sediments and the consequent destruction of the 

 remains. 



There is another fact which must be considered in this connection, 

 namely, that the abundant inpouring of fresh water into the lagoon may 

 have at times transformed it into a brackish or even fresh water body. 

 The Gulf of Finland is almost a fresh-Avater body, especially near its 

 head, while the Baltic as a whole has a surface salinity of 7.8 permille, 

 though the volume salinity is 10 permille. Compared with the normal 

 salinity of sea-water, 35 permille, the influence of abundant impouring 

 of fresh water is at once seen. During an earlier period the sea was 

 entirely excluded from the Baltic and the fresh-water Ancyclus Lake 

 replaced the Baltic. After the cutting off of the head of the Gulf of 

 California by the building of the Colorado delta, the abundant inpouring 

 of fresh water by that river converted the original salt body (with oceanic 

 salinity of 35 permille or over) into a fresh-water lake of greater eleva- 

 tion and extent, in which an abundant fresh-water fauna existed. The 

 diversion of the river water to the gulf, and possibly an increase in 

 aridity, caused the fresh-water lake to concentrate once more into a salt- 

 water body, this time, however, isolated and probably lifeless. Finally, 

 through continued desiccation, the present depressed desert was pro- 

 duced, with the concentration of the salt in the center of the old sea- 

 bottom. The influence of streams on the salinity is shown in the Black 

 Sea, which has a salinity in the upper 40 or 45 meters of 18.3 permille, 

 while the Sea of Marmora has a surface salinity of 22 to 25 permille and 

 the Mediterranean the normal of 35 permille or over. The Sea of Azov, 



