loo The American Geologist. February, looi. 
one here suggested can be advantageously or safely adopted is 
a matter for discussion. Are the molar, or, it might be termed, 
the molecular features of a fauna indication of comparative 
age? In any case it would be merely an auxiliary consider- 
ation. It might be easily misapplied and warrants in itself 
no ultimate decision. But is it not helpful? 
Is it reasonable upon this suggestion to place the Spergen 
Hill beds, which, as Whitfield has said, "contain an interming- 
ling of species known to occur in the Keokuk, St. Louis, and 
Chester limestones," below any of these from the immature 
condition of size and development of these fossils? By no 
means. The whole expression of the Spergen Hill fossils, as 
found in the limestone beds is one of repression. These depaup- 
erate forms have resulted apparently from a peculiar fecun- 
dity, or tidal concentration,which has, under some unfavor- 
able conditions, produced a great number of interfering indi- 
viduals. At Poynter's Hill and at Ellettsville, at no great dis- 
tance from Spegen Hill, these same beds, with fewer fossils, 
show larger individuals. 
But, on the other hand, it would seem reasonable to con- 
clude that the western Hudson River, which as Hall long ago 
observed, contains a fauna of brachiopods, with corals, crin- 
oids, Crustacea and trilobites, and in the same species far more 
developed than the fossils of its eastern equivalents, was later 
than the New York beds of the same age. 
(To he Continued.) 
THE PLAN OF THE EARTH AND ITS CAUSES* 
By J. W. Oregoky, IJ.Sc, Melbourne, Aus. 
Tin-: Variation of Topographic Form. 
L)cs])ite the cxtroiiie variability in the shapes of the con- 
tinents and their apparently capricious distribution, geogra- 
phers of all ages have lielieved that the arrangement of land 
and water on the globe is based on a regular plan. The plan 
can. of course, only be recognized in broad outline, for the 
shape of the land-masses depends on the structure of the 
Earth-forms, which vary indefinitely. Intricate mountain valley 
* Prom 'The Ceo^raphical JnitrnaV for March. 
