154 
IOWA ACADEMY OP SCIENCES. 
breaks in sedimentation, or the leading cycles of mountain- 
making activity. These are essentially the horizons of uncon- 
formities, and they are extended laterally across as much 
territory as they approximately affected. A large number of 
less important unconformities are known. The whole, when 
thus arranged, forms an interlocking series of absolute datum 
planes by which may be paralleled all geological sections. 
The present scheme is based upon our present plan of 
geological chronology. In the main this is unchanged, though 
there is, doubtless, a considerable element of error that will 
have to be eliminated as the more exact determinations of 
parallelism are made out. The larger divisions or systems 
may be left very nearly the same as they are now. The minor 
subdivisions which cannot now be brought into juxtaposition, 
can readily be placed in the general scale. This appears to be 
one of the advantages recommending such a scheme. 
A PRELIMINARY LIST OF THE MOSSES OF IOWA. 
BY T. E SAVAGE. 
The mosses together with the Hepaticas or liverworts con- 
stitute the group of plants known as the Bryophytes. This 
group is distinguished from the Thallophytes, by the fact that 
they present two modes of reproduction, the sexual and the 
asexual, which occur in regular alternation. This gives rise to 
what is called alternation of generations. Most bryophytes 
also exhibit a very fair differentiation as between stem and leaf. 
The spore of the moss, on germinating, produces a many- 
celled, branching filament containing chlorophyl, the protonema. 
From the protonema are developed colorless rhizoids, which 
penetrate the substratum, and buds which produce the stem or 
leafy axis of the plant. At the apices of the stems, or of the 
small lateral branches, are borne the sexual organs, the 
antheridia and archegonia. Mosses may be monoecious, the 
antheridia and archegonia being produced on the same plant, 
or dioecious, the sexual organs being borne on separate plants. 
The protonema and the leafy stem with the sexual organs 
make up the sexual generation. 
