CORRELATION OF STRATA. CV 
bed becomes thicker and more argillaceous, taking it as a whole, and contains molluscan 
species of the genera Clenodontu, Clidophorus and Orthoceras in increasing abundance. 
Resting upon the Utica we find in Fillmore county from 10 to perhaps 25 feet of more 
or less thin bedded argillaceous and siliceous limestones belonging to the Richmond group. 
Some of the layers are full of fine fossils, chiefly Brachiopoda, and these are often silicified, 
in which condition they have been collected by hundreds near Spring Valley. With very 
few exceptions, all the fossils that have been found in these layers occur also in the 
Richmond group of Illinois, Indiana and Ohio. 
The strata of this group in Fillmore county are quickly decomposed and covered with 
soil, so that satisfactory natural exposures are rare. The fossils may sometimes be picked 
out of the worn soil of old fields but a more abundant supply was obtained in the cuttings 
along the railroad between Wykoff and Spring Valley. A few of the characteristic species 
are Orthis subquadrata, O. proavita, O. testudinaria (large variety), O. whitfieldi, Rafinesquina 
kingt Whitfield sp., Rhynchonella capax, Strophomena neglecta, Batostoma variabile, Cteno- 
donta similis, C. recurva and Streptelasma rusticum. 
Overlying the fossiliferous layers of the Richmond group, may be seen in two places 
near Spring Valley, one about two and one-half miles north, the other one mile east of 
the town, about six feet of sandy layers weathering into irregular lumps and thin shells. 
Some of these contained fragments of large crinoids or cystids, and from the loose material 
we obtained several fine examples of Hindia spheroidalis, a common Upper Silurian 
fossil; also spicules of Hyalostelia solivaga which occurs nearly everywhere in connection 
with the Hindia. Though these six feet are probably to be regarded as Upper Silurian 
the passage lithologically from the Richmond group is exceedingly gradual. 
Succeeding the foregoing bed and followed with not very strong evidences of uncon- 
formity by Devonian strata, is a sandstone four feet thick which here and there contains 
large numbers of small quartz pebbles, varying between one and ten mm. in diameter 
This sandstone we assume to belong to the Oriskany of New York. 
TABULATION OF THE LOWER SILURIAN SPECIES OF MINNESOTA AND GENERAL 
REMARKS INTRODUCTORY TO SAME. 
In the following tables the student will find the names of all the species known to 
occur in the Minnesota strata of the Trenton and Hudson River periods, excepting the St. 
Peter sandstone. To these are added a number that are likely to be found within the 
limits of the state but are as yet known to occur only in the neighboring states of Wis- 
consin, Ilinois and Iowa. With these the total number of forms catalogued is 809. Species 
described in the volume from other regions are not included in the list since they have no 
bearing at present upon the points which the tables are intended to bring out. These 
tables show also the stratigraphic and geographic distribution of the species, while 
the summary tables which follow the list show how the faunas of the various stratigraphic 
divisions compare with each other and with those recognized in the Cincinnati, Tennessee, 
New York and Canadian Lower Silurian regions. 
