PROCEEDINGS. 
537 
lighter matter above and the heavier beneath must slope toward the 
land. When isostacy is disturbed by degradation on the land side or 
deposition under the water, the tendency to flow landward in restoration 
of equilibrium cannot reside in the superficial portion, because it would 
there be opposed by gravitation. It must be in the deeper, denser ma¬ 
terial, so deep that it is difficult to understand its crumpling effect on 
the surface layers. 
In response to Mr. Gilbert, Captain Dutton said that there is no limit 
in depth to the flow. Owing to the higher rigidity of the surface rocks 
the flow would be greatest at great depths, but would extend to near the 
surface, and by the adhesion of the parts of the viscous mass the surface 
portion would be carried along and crumpled in much the same manner 
as the rough surfaces of certain lava flows. He cited the case of the 
wrinkles on the surface of “ pahoehoe ” in the Hawaiian lavas as exam¬ 
ples and illustrations on a small scale of the effect of a more rigid surface 
in a less rigid flowing mass beneath. 
Remarks were also made by Mr. Woodward, who opposed the view 
that secular contraction plays an unimportant part in crumpling the 
earth’s crust. While not unmindful of the difficulties of the contraction 
hypothesis, he considered it an essential basis for the hypothesis of isos¬ 
tacy. The process of isostacy tends at a relatively rapid rate toward 
equilibrium ; it ought, apparently, to run down in a comparatively brief 
geologic age. The process of contraction goes on at a relatively slow rate 
and is continually opposing the equilibrium to which isostacy leads. 
Both processes tend to produce crumpling along lines of weakness, and 
though that of isostacy may have been the more effective of the two, it 
appears to require secular contraction for its maintenance. 
Mr. Woodward called attention also to Dr. Helmert’s recent memoir on 
plumb-line deflections, in which it is shown that the observed deflections 
in the vicinity of the Appalachians are on the whole toward the moun¬ 
tains rather than from them. 
Mr. Walter Harvey Weed read a paper on The Formation 
of Deposits of Lime, Iron, and Silica by Plant Life. 
[Abstract.] 
It is a little strange that the chemical and geological work performed 
by plant life has not been recognized by naturalists who have studied so 
carefully the analogous work of the mollusks, corals, and other forms 
of animal life. It has long been known that certain marine algae build 
stony structures of carbonate of lime, and more recently that certain 
mosses and fresh-water algae are lime-encrusted because of the vital 
activity of the plants. My own observations upon the subject convince 
me that the importance of the subject is not realized, and that the 
deposits formed in this way are not only sometimes of great magnitude, 
as is the case at Tivoli and Carlsbad, and more especially in our own 
69—Ball. Phil. Soe., Wash., Vol. 11. 
