108 = Review of Hail and Whitney's Geology of Iowa. 
organic life abundantly disseminated through the rocks, a lat 
tween the compact calcareous beds, are crowded with forms oe 
identical with, or closely allied to, those which char : 
renton limestone in its extension from New York t Michi: 
Canada and on the northern shores of Lakes Huron and a 
gan, and as far west as the Mississippi, along a line of oute™ 
some 1500 miles in extent, - mestoe 
Galena Limestone.—The passage from the Trenton lim ab 
into the next succeeding member of the series, the Galena 1 
Stone, is not an abrupt one; on the contrary, there are, in My 
localities, several alternations of caleareo-magnesian and pute? 
calcareous layers, indicating that the change of conditions W" 
resulted in the deposition of the highly crystalline pan 
which overlies the Trenton was not effected at once, nor W! The 
occasional partial returns to the former state of things: 14 
Galena limestone, as usually developed, is a rather thick-beatt 
light greyish, or light yellowish grey, dolomite, distinctly rig 
in its texture and usually rather coarse-grained. 7) 
Tegularity with which it weathers, leavin icturesque outleD 
wit Castellated forms, like watch-towers, oF the half-ruined a 
of ancient fortified cities, The quantity of insoluble matte” 
