Geology of the St. Croix Dalles. — Bcrkcy. 141 
Franconia sandstone. The few fossils which occur in lim- 
ited horizons are casts from which all traces of the original 
shells have been removed. A peculiarly regular distribution 
of an iron compound, although the forms outlined by it can- 
not be identified, is believed to indicate the position of some 
soft-bodied forms, perhaps plants. Copper minerals are not 
uncommon, but are always in small (juantities and are usually 
in the form of carbonates. 
The green-sand and shale member, the Upper Dresbach. 
is characterized by a bright or dark green to a greenish gra}- 
color. The comparatively large and more evenly roundetl 
quartz grains in the upper portions of the subdivision, the 
abundance of glauconite in the quartzose beds, a shaly and 
thin-bedded appearance of exposed portions and a generally 
friable nature are typical of these beds. An abundance of 
broken but undecomposed shells, an occasional development of 
calcareous cementation, cross-bedding in the green-sand bed 
and the occurrence of several accessory minerals both orig- 
jnal and secondary- — altogether make the green-sand and shale 
member a characteristic one. Occasionally large pebbles of 
quartz are fovnid, and in one instance a pebble of quartzyte 
was observed. Travertine, apatite crystals of microscopic 
size, copper carbonate and iron oxide stains constitute the 
principal accessory minerals. 
The calcareous and pyritiferous shales, called Lower Dres- 
bach, are distinguished from the middle member by differ- 
ences which to some extent are the result of the peculiar local 
conditions under which they were formed. They occupy the 
long, narrow, pre-Cambrjan valley between two diabase ridges 
now followed by the St. Croix river north of the Dalles. This 
valley probably became a secluded bay on the margin of the 
Cambrian sea coast in whose shelter myriads of animal forms 
found a favorable environment. Among the most pronounced 
characters of this member are: — the highly calcareous content 
which in many places develops numerous layers of limestone 
from one to three inches in thickness throughout a vertical 
range of 40 or 50 feet; an abundance of fossil remains of the 
Lingulepis type furnishing a plentiful supply of the carbon- 
ates; and a development in this shale of secondary concretion- 
ary pyrite, which furnishes by alteration the sulphates of iron 
