338 J. W. Dawson— Erian Flora of the United States. 



I was led by this result to consider the case of the earth -moon 

 i (as I believe they did) from the central solar mass, in 

 the form of a swarm of discrete masses of meteoric iron and 

 stone, each one having the temperature of the cold of inter- 

 stellar space, or not much above it. Translating tins concep- 

 tion into ms age, I find that the equation of 

 continuity belonging to the hydrodynamical theory applies 

 equally well to the meteoric theory, viz : 



vy = v'y' (7) 



here v, v', are the velocities at any two points, and y, y' are the 

 ths of the ocean or meteoric swarm at the same points. 



depth 



Tin 



s depth of the swarm or ocea s; or friction 



will be least under the moon, and greatest at right angles to the 



moon, and the velocities will be inversely. Hence the cbanoea 



be meteorites, when disturbed by the moon's 



.a! to the velocity, being; greatest 



where the velocity is greatest and th< u a .of passage h -1 ai 1 



This consideration reduces the meteoric problem to that of 

 the hydrodynamical problem, with a friction proportional to 

 the velocity, and gives equations, in all respects similar to those 

 derived by Mr. Darwin, from the hypothesis of a viscous earth. 



On the meteoric hypothesis, if the jostling of the stones be 

 slow they may cool almost as fast as they are heated and the 

 result will be" a cool earth and almost indefinite time at the 

 disposal of geologists. 



The following notes are extracted from a Eeport on Devonian 

 Plants prepared for the Geological Survey of Canada, but not 

 yet published. They relate to the recent discoveries made in 

 the United States, and their bearing on points of interest with 

 reference to the Erian Flora. 



I. The Nature and Affinities of Ptilophtton. 

 {LycopoclitesVanuxemii of Report on Devonian and Upper Silu- 

 rian Plants, Pt. I, p. 35.— L. plumida of Report on Lower 

 Carboniferous Plants, p. 24, PL I, figs. 7, 8, 9.) 

 In previous publications these remarkable pinnate frond-like 

 objects were referred to the genus Lycopodiies, as had been done 

 by Gceppert in his description of the European species Lyco- 

 podites pennceforrnis, which is very near to the Americ 

 form. Since 1871, however, there have been many new specimens 



