Decembee 18, 1896.] 



SCIENCE. 



919 



The weight of precedent and usage, as 

 shown by the eminent authorities cited, un- 

 doubtedly assigns the Wealden beds to the 

 base of the Cretaceous. In view of this al- 

 most unanimous opinion of the ablest indi- 

 vidual authorities, the established usage of the 

 official surveys of Great Britain and the United 

 States, and the expression of the representative 

 International Congresses of Geologists, to the 

 eflfect that the Wealden is the base of the Cre- 

 taceous system, it will require more than the 

 assertion of one savant, however eminent in 

 research, to change the accepted geologic classi- 

 fication. At least some preconcerted action 

 and international agreement on the part of the 

 geologic societies should be had before one 

 would be justified in using the broad term 

 Jurassic for beds which at the utmost could 

 only doubtfully be referred to it, and which, if 

 so referred, Avould represent only a minute 

 fraction of the great time period of the Jurassic. 



Periods of geologic time usually correspond 

 with one or more great oscillations of land and 

 accompanying migration of the marine shore 

 lines, producing cycles of sedimentation. These 

 cycles are recorded by successive variations in 

 the character of the sediments manifested: 

 First, by estuarine and other unsorted marginal 

 deposits, representing the beginning of the sub- 

 sidence. In turn these are succeeded by more 

 finely sorted and deeper-water or off-shore 

 beds, as subsidence of bottom and landward mi- 

 gration of shore progresses. The Wealden in 

 England and the Potomac in America most 

 clearly represent the basement littorals of the 

 Lower Cretaceous epochs of sedimentation, be- 

 longing by every physical affinity and grada- 

 tion with the overlying beds. 



Furthermore, the Potomac from New Jersey 

 nearly to the Rio Grande is undoubtedly a 

 marginal, land-derived formation, laid down at 

 oceanic deposition level, and one which marks 

 the initiation of the great cycle of Lower Cre- 

 taceous sedimentation, recording the encroach- 

 ment of the Cretaceous sea upon the pre-exist- 

 ing Jurassic continent. In Texas these beds 

 certainly lie unconformably alike upon Al- 

 gonkian, Silurian, Carboniferous, Permian and 

 alleged Triassic, and there is not a trace of 

 pre-existing Jurassic sediments. 



The third method, based upon the presence of 

 characteristic fossils, is likewise valuable, but 

 least trustworthy. The land and fresh-water ani- 

 mals, land plants and marine mollusks each pre- 

 sent a widely varying standard, and leave 

 room for differences of opinion upon the part 

 of their respective students. While each of 

 these (except the fresh-water mollusks, which 

 seem of little diagnostic value) has peculiar 

 characteristics for each of the great periods, 

 there is no reason to presume that research may 

 not often lead to the discovery of the persistence 

 of supposed characteristic Jurassic forms into the 

 Cretaceous, or Cretaceous forms into the Terti- 

 ary, as has been done in some instances. In 

 such cases, however, no one has ever changed the 

 period designation of the beds. For instance, 

 ammonites were once reported to be found 

 in the Eocene of India, but no one has trans- 

 posed the Eocene epoch from the Tertiary to 

 the Cretaceous on that account, and even if 

 Prof. Marsh has found Jurassic land vertebrates 

 in the Wealden it is doubtful if he would be 

 justified, in face of the opposing evidence of the 

 plants, mollusks and sediments, in making such 

 a radical step as transferring the Wealden beds 

 from the Cretaceous to the Jurassic period. It 

 would be far more logical, in my opinion, to con- 

 sider that the vertebrate life of the Jurassic 

 land has persisted slightly into the Cretaceous 

 period. 



Regardless of European analogy, however, 

 there is every stratigraphic and paleontologic 

 reason for placing the Potomac-Trinity forma- 

 tions of America as the base of the Cretaceous. 

 In Texas, plants, vertebrates and marine mol- 

 lusks are found associated in the basement 

 Trinity beds, the equivalent of the lowest Poto- 

 mac. The plant life and molluscan life which 

 have been most thoroughly studied show no 

 more Jurassic characters than are usually found 

 in these basement beds. 



With all due deference to the opinion of 

 others, there are reasons for suspecting that no 

 marine Jurassic formations of Atlantic sedimen- 

 tation have as yet been discovered north of 

 Argentina on the present Atlantic slope of the 

 American hemisphere, and furthermore I hold 

 that there are strong reasons for believing that 

 this absence is due to the fact that the conti- 



