286 The American Geologist. Aprii,i894 
respond to the basal planes of the staurolite. and whose apices join a1 
the center of the crystal, while from the acute and obtuse edges of the 
pyramids the inclusions extend as films or tins to the vertical edges of 
i he prism. G. 
Continuity of the Glacial Period. By G. Frederick Wright. Am. 
.lour, of Science, 111, vol. xi.vn. pp. 161-187, with sections and five maps; 
Ma rcli. ls«.) 4. 1 1 is proved, as the author tli inks, by his observations and 
those of Mr. Leverett and others, thai the erqsion of tin- rock gorges of 
the Ohio river and its tributaries was preglacial, instead of being inter- 
glacial, which last has been their interpretation by Prof. Chamberlin. 
It is further urged thai this removes the necessity for the belief in a 
long interglacial epoch, and that it appears more in accordance with all 
the facts known to ascribe the glacial drift to a single and continuous 
Ice aye. On very slender evidence Mr. McGee has estimated that an 
epoch of warm interglacial climate in northeastern Iowa extended 
through probably 200,000 and perhaps 2,000,000 years: but the forest 
bed enclosed between deposits of till, on which his opinion rests, is 
thoughl In Prof. Wrighl to be explainable by short retreats and soon 
returning advances of the ice-sheet, with a forest at its border, as in the 
case of the Malaspina glacier in Alaska. 
CORRESPONDENCE. 
( Iomposite Generic Findamknta. Some remarks on "Mathematical 
Biology" in the February number of •"Natural Science," and the sug- 
gestion madebj Bather in his recent work on the "Crinoidea of Gotland," 
that Galton's method of composite portraiture mighl be employed to 
ascertain a specific type from which variations in various degrees could 
lie reckoned, recalled to me some germane experiments in which 1 was 
interested some eighl years ago, and which indicate that there is even 
a possibility among narrowlj restricted, up-to-date genera, of educing, 
in a series of comparative measurements and successive overlays, a sort 
of generic fundamentum. I take the liberty of citing and illustrating a 
single instance of this among the fossils, the case being a simple one and 
involving the search for the standard in one element of variation only, 
namely, outline; but whatever maj be done with a single specific char- 
acter, in t his manner, is equally possible of all the others. 
In volume v. pt. i. of the Palaeontology of New York are described •">; 
species of the lamellibranch genus Leptodesma, two of which are as- 
cribed to the horizon of the Hamilton group, and 55 to the fauna of the 
overlying Chemung beds. Leptodesma is an aviculoid genus with 
smooth and convex valves, and the shells upon which these observations 
are based, are preserved plump ami without disturbance of the margi- 
nal outline. The genus in some of the higher beds of the Chemung 
group is enormously prolific, so that the perception and identification of 
