254 
STRATIGRAPHICAL GEOLOGY 
Trenton stratum. In his last word on this subject Calvin ® con¬ 
siders the Dubuque section alone; and designates all beds, whether 
mineralized dolomites or normal barren limestones, which lie 
between the blue, Maquoketa and the green. Decorah shales as 
Galena Limestone. This happens to be very nearly Hall’s position 
in. the beginning. ^ 
Nowhere, therefore, in all the geological literature relating to 
this region does there appear to be a single instance in which the 
title Galena Limestone is used in a strictly geographic sense, 
with a definite type-section noted, and a distinctive fauna de¬ 
limited. The original mineralogical significance undimmed, per¬ 
sists to the present day. 
In view of the fact that several terranal units are now clearly 
differentiated from the old Galena-Trenton formation, that a 
distinct formational unit is established which embraces both 
normal blue layers and their altered dolomitic phases, and that ac¬ 
cording to modern canons of geological nomenclature such strati¬ 
graphic units should be fixed by geographic titles, it appears nec¬ 
essary, notwithstanding its long usage, to discard the old min- 
eralogic term Galena as a terranal name. It is therefore pro¬ 
posed to apply to the formation lying immediately above the 
Green, or Decorah, shales a geographic title derived from some 
Dubuque locality where the terrane is displayed in typical section. 
Julian Limestone is suggested—from the township in which the 
city of Julian Dubuque is situated, and where at the famous 
Eagle Point the most accessible exposure is characteristically 
shown. 
In their relations to the dolomite and the normal limestone the 
life-zones of the Julian formation are particularly instructive. 
Some of them are incidentally recognized by Calvin. The Green, 
or Decorah, shales underlying the terrane under consideration 
are especially characterized by an Orthid fauna. Two species in 
particular, Orthis suhquadrata, Conrad, and O. tricenaria, Con¬ 
rad, are prolific in their occurrence. One hundred and sixty feet 
above the base of the formation is a zone 10 feet in thickness 
which is unusually rich in gasteropods. Thirty feet higher is 
another 10-foot zone which is composed largely of rhizopod re¬ 
mains — the gigantic Receptaciilites. Both of these prominent 
Qlbid., Vol. XVII, p. 192, 1907. 
