38 
llie American Geologist. 
January, 1895 
the north this interval of 80 feet might become 50 feet, or the 
overl3dng limestone might entirely disappear, while toward the 
south it dwindles and disappears, allowing the base of the Ga- 
lena limestone proper to approximate the lowest limit of the 
characteristic fauna. 
While the foregoing indicates an intimate connection be- 
tween the Trenton and the Galena, a closer examination of the 
faunas which are found above and below this arbitrary line, 
throws still more evidence on their alliance. A preliminarj^ 
partial tabulation of the results of our study appears below. 
Hudson Rivek, Gai.exa and Tkenton faunas. 
re 
S 
. s 
So 
'o'ln 
H 
0) 
°g ■ 
'ptTl 
Total Species. 
(With varieties.) 
Species belonging 
TO TWO periods. 
Classes. 
Hud- 
son 
River. 
Galena. 
Tren- 
ton. 
Hud- 
son 
River 
and 
Galena. 
Galena 
and 
Tren- 
ton. 
Hud- 
son 
River 
and 
Tren- 
ton. 
7 
2 
4S 
2i 
27 
11 
21 
s 
3 
10 
145 
".> 
in 
62 
32 
2 
2 
2 
I 
27 
iS 
6 
7 
5 
I 
% 
41 
15 
-3 
5 
2 
Graptolites 
7 
119 
37 
60 
49 
15 
........ 
4 
11 
1 
I 
8 
Brachiopoda 
Lamellibranchiata. 
I 
Trilobites 
2 
Totals 
JS7 
446 
65 
1.S7 
292 
9 
43 
4 
It appears therefore that forty-three species are known to 
be common to the Trenton and the Galena, in Minnesota, 
and but nine common to the Galena and the Hudson Eiver, 
four of those nine being such hardy species as to survive 
from the Trenton to the Hudson river. This indicates that if 
the break which has been found by Mr. Walcott to prevail 
so widely over the country, separating the Trenton from the 
Utica slate, be sought for in the northwest, it cannot exist be- 
tween the Trenton and the Galena, but must be looked for at a 
higher horizon. 
Prior to Mr. Walcott's generalization it had been customary 
to place the possible Utica horizon with the shale overlying 
the Galena, i. e., in the Maquoketa. Prof. James Hall called 
attention to a certain black shale at the base of the Maquo- 
keta, in Iowa, in 1858, as the probable western representative 
