Ulrich—Paleozoic Systems in Wisconsin. 
89 
faunaj referred to and discussed by Ulrich in 1916d^ These addi¬ 
tions—in all 10 species, most of them obviously primitive gastro¬ 
pods—make up what may be called a distinguishable but unde¬ 
niable preoccurrence of species typically developed in the much 
younger Lower Ozarkian Mendota dolomite. Many even more 
striking instances of recurring faunas are now known, so that no 
particular importance is to be attached to this at first perplexing 
case. 
In southern and western Wisconsin the zone of the St. Lawrence 
limestone is seldom absent. Where present it is always more limy 
than the superjacent and interjacent beds; and as a rule it is un¬ 
questionably indicated by thick, often quarried layers of character¬ 
istically colored crystalline magnesian limestone. No other bed 
that resembles it at all closely occurs in the prevailing sandy Upper 
Cambrian series in the Upper Mississippi Valley. However, it 
varies greatly in thickness from place to place. In the vicinity of 
Sparta it usually is only about 2 feet thick, between Mazomanie and 
Black Earth 3 or 4 feet, at Spring Green about 6 or 7 feet, at 
Pheasant Branch (west end of Lake Mendota) 12 feet, at Farwells 
Point, on north shore of Lake Mendota, only 2 feet. Apparently 
it is entirely absent to the north and east of the Baraboo Range. 
In the bluffs along the Mississippi its development is variable. It 
was not observed in the section at La Crosse, but to the north of 
that place, as far at least as Stillwater, Minn., its zone is generally 
recognizable. 
Basal shale .—^The base of the Trempealeau is often a shale or 
shaly sandstone with considerable greensand and occasionally thin 
layers of dolomitized sandstone. At the bottom usually there is a 
thin layer of sandstone conglomerate. In thickness it varies from 0 
to 15 or even 20 feet, the latter maximum being found on the east 
side of Blue River Valley about 4 miles southwest of Muscoda. 
This place also is the only one at which satisfactory fossils were 
procured from this shaly bed. The fauna as indicated by this 
collection of seven or eight species comprises three or four seen 
nowhere else and two small species of Saukia that can be compared 
only with S. pyrene and S. leucosia, two species known elsewhere 
only in the Norwalk member of the formation. 
It is not at all certain that the shaly bed at the base of the 
Trempealeau is always of the same age as the one found in this 
Correlation by displacements of the strandline. Geol. Soc. America, Bull., vol. 27, 
p. 477, 1916. 
