CORRELATION OF STRATA. Ixxxvli 
2. Twelve feet of shelly or thin-bedded argillaceous limestone, the surface of the layers, in the 
lower half especially, being rough. Near the middle several large impressions of Receptaculites oweni, one 
quite fifteen inches in diameter, were noticed. Fossils are neither very plentiful nor well preserved in 
these layers. 
3. Twenty-nine feet of rather thin-bedded, compact, bluish-gray, limestone, the purity of the 
limestone increasing from below upward. Contains numerous fossiliferous layers, the fossils being chiefly 
Brachiopoda of the family Strophomenide. Of other forms a fine new species of Palcwocrinus deserves 
mention. 
4, Nine feet of cherty limestone. Fossils abundant, Orthis tricenaria, O. plicatella, Strophomena 
billingst, Clitambonites diversus, Parastrophia hemiplicata, and some branching monticuliporoids. 
5. Fifty-six feet of fine-grained and subcrystalline limestones; some argillaceous layers, thinner 
than usual, in the upper twelve feet. About eleven feet beneath the top we find a layer full of graptolites, 
probably of the genus Diplograptus. Above this layer fossils are comparatively rare, but beneath it they 
are abundant though rather difficult to obtain perfect, since they must be broken out of the solid lime- 
stone. Among others weobtained here Rafinesquina deltoidea, Plectambonites gibbosa, Zygospira uphami, 
Ambonychia bellistriata, Byssonychia intermedia, Clionychia undata, Ctenodonta intermedia, Cyrtodonta abrupta, 
C. germana, Endodesma cuneatum, EH. compressum, Psiloconcha? minnesotensis, Eccyliopterus owenanus 
Fusispira inflata, F. nobilis, F. planuiata, Trochonema robbinst and Platymetopus robbinsi. 
Above No. 5 this ravine exposes about fifty feet of massive dolomitic limestones of the Maclurea 
bed and then about twenty feet of shaly layers belonging to the Utica group. 
Section 5, showing strata as seen in and near a small quarry, about two and a half miles north of Spring 
Valley, Minnesota. 
: 1. About nine feet of thin and unevenly bedded or laminated, bluish-gray, crinoidal limestone. 
The whole appears solid in a fresh exposure but breaks up under the weather. The crinoidal fragments 
have evidently been much rolled. 
2. Twenty feet of even bedded, compact, gray limestone, in layers fifteen inches or less thick, 
the layers becoming too thin and argillaceous in the upper part for building purposes. Between the lime- 
stones there are bands of soft shale, the whole bed being composed of about one-third shale and two- 
thirds limestone. At the top several layers will be noticed containing the separated parts of Asaphus or 
Tsotelus maximus in abundance. Six feet beneath the top a band of shale contains Leptobolus occidentalis, 
three species of Lingula and Climacograptus typicalis (?). Beneath this there is another band from which 
Orthis testudinaria, varieties multisecta and emacerata, and two discoidal species of Bryozoa were obtained 
in considerable abundance. The same layer afforded also Triplecia ulrichi, Plectambonites praecosis (Sarde- 
3on) and Calymene callicephala var. mammillata. 
3. These are covered at this locality. About a mile east of Spring Valley they are exposed as 
thin bedded, arenaceous and argillaceous limestones; some of the layers are full of Brachiopoda and other 
fossils characterizing the Richmond group of the Cincinnati region. 
4. Arenaceous strata, six feet thick, weathering into irregular lumps, some of which contain 
plates and columns of large crinoids or cystideans, and Hindia spherodalis. Probably Upper Silurian. 
5. Four feet of rather coarse sandstone, including here and there an abundant supply of smal] 
quartz pebbles. 
6. Above No. 5 the surface of the ground was strewn with irregular, porous lumps of yellow 
or buff, magnesian limestone of Devonian age. 
CORRELATION OF STRATA. 
TRENTON PERIOD, 
Chazy group. 
St. Peter Sandstone. A number of fossils have been found in this well known division 
of the Lower Silurian, but they are nearly all very ill preserved, the nature of the sediment 
being unfavorable. As they have not been included in this report, it may bé well to say 
of them that, as far as their condition will admit of judgment, they are of types reminding 
one aS much, perhaps, of species characterizing the Stones River group as of Chazy 
forms. It is, however, as a part of the latter group that the St. Peter is to be viewed. 
