918 



SCIENCE. 



[N. S. Vol. IV. No. 103. 



of mine published in Science, appears to con- 

 vict me of inconsistency and to derive the con- 

 clusion that certain subjects introduced in an 

 illogical manner into his ' Elements of Geom- 

 etry ' find a satisfactory treatment in his ' Ele- 

 mentary Synthetic Geometry.' These subjects 

 are the straight line as a minimum length and the 

 general notion of the length of a curved line. As 

 a matter of fact, neither of these subjects is 

 discussed in the latter work. The only curved 

 line there considered is the circle. 



Thomas S. Fiske. 



a question of classification. 



To THE Editor of Science : The communi- 

 cations by Profs. Hollick and Ward in your 

 recent numbers, commenting upon Prof. Marsh's 

 determination of the Jurassic age of the 

 Potomac and Amboy clays of the south 

 New England island series and New Jersey- 

 Virginia coast, and Prof. Marsh's reply to Prof. 

 Hollick in the November number of the Ameri- 

 can Journal of Science, are of interest to all 

 workers in American Mesozoic formations. 



Prof. Marsh, in his early papers on the verte- 

 brates of the Atlantosaurus beds of the age of 

 Colorado and the Potomac beds, has referred 

 both of these to the Wealden epoch. Ameri- 

 can geologists * do not assert the existence, in 

 this country along the present Atlantic slope, 

 of Jurassic beds of Atlantic sedimentation rep- 

 resenting the whole or part of that vast period 

 of time below the Wealden. In view of these 

 facts, it is no inference to state that the broad 

 generic term ' Jurassic period, ' as applied by 

 Prof. Marsh to this portion of our country, is a 

 synonym for the term Wealden epoch, as used 

 by others. Hence that part of the controversy, 

 so far as it involves the oldest or Potomac beds, 

 narrows down to the question of whether the 

 beds of the Wealden epoch should be classified 

 as the top of the Jurassic or the base of the 

 Cretaceous period of geologic time. 



*With the exception of Prof. Jules Maroon, who 

 originally maintained that the Middle and Lower 

 Cretaceous of Texas and the Plains Tertiary were 

 Jurassic, and who still maintains the Jurassic age of 

 the Middle Cretaceous beds of New Mexico and the 

 Lower Cretaceous of Texas. This position has been 

 disproved by research. 



This is an old and much discussed question 

 of English geology. It would be impossible 

 here to give even brief reference to the exten- 

 sive literature of the question. It is sufficient 

 to say that, after the most thorough sifting of 

 the evidence pro and con, European opinion 

 and usage of to-day uphold the Cretaceous age 

 of the Wealden beds. Against the opinions of 

 a few who hold to the contrary, a volume could 

 be filled with the data of eminent European 

 authorities who maintain the Cretaceous age of 

 the Wealden, including L. Agassiz, Lyell, 

 Jukes, Prestwich, Zittel, Etheridge, Woodward, 

 Pavlow, Fischer and others. Even as I write 

 these lines the mail brings, fresh from the press of 

 the official Geological Survey of Great Britain, 

 an elaborate monograph of five volumes on the 

 English Jurassic by H. B. Woodward, which 

 excludes the Wealden from the Jurassic and 

 places it at the base of the Cretaceous period. 

 Furthermore, the consensus of opinion in all 

 the reports of the meetings of the International 

 Congress of Geologists places the Wealden as 

 the base of the Cretaceous system. 



Prof. Marsh, however, has assumed the posi- 

 tion in several writings that the Wealden epoch 

 belongs to the Jurassic period and not to the 

 Cretaceous, and this opinion explains his use of 

 the term Jurassic in this country. On the other 

 hand, all the able authorities of this country, 

 except Prof. Marsh, who have studied or re- 

 viewed the Potomac and allied formations of 

 Wealden affinities have reached the final con- 

 clusion that they are of Cretaceous age. Among 

 these may be mentioned Dana, Newberry, 

 Ward, McGee, Hollick and others. The U. S. 

 Geological Survey has also mapped these for- 

 mations as Cretaceous upon its latest atlas 

 sheets. 



The controversy, in part, thus narrows down 

 to the problem of the age of the beds of 

 the Wealden epoch, and naturally arouses an 

 inquiry as to what criteria can be depended 

 upon to settle the limitations of the geologic 

 periods. There are three plausible methods 

 that suggest themselves : (1) precedents and 

 usage ; (2) correspondence of the rock of 

 each period with great cycles of sedimentation, 

 and (3) the presence of characteristic, distin- 

 guishing fossils. 



