176 GENERAL GEOLOGY. 



The State collections contain some specimens of these brick, 

 which show that the clay is excellent for that purpose. An 

 analysis of the clay will be found in the report of Prof. 

 Emery in volume two. 



Fossils. Hitherto, with the exception of the traces of 

 crinoidal stems found in the Lower Magnesian limestone 

 at McGregor, we have found no fossils in the formations 

 which precede the Trenton limestone; but in this last named 

 formation fossils are extremely abundant. So much so in 

 some places, indeed, that the rock is made up of a mass of 

 shells, ' corals, and fragments of trilobites, together with 

 other animal remains, cemented by calcareous material into 

 compact rock. It would appear that the waters in which 

 the preceding formations were deposited were comparatively 

 free from life, but upon the introduction of the Trenton epoch 

 its seas swarmed with life as various in its expressional 

 forms as that of the seas of to-day. Among these fossils found 

 in the Trenton limestone of Iowa, are those well-known 

 species which characterize the formation in other parts of the 

 country, besides some that are yet new to science and 

 peculiar to Iowa s and neighboring strata. 



THE GALENA LIMESTONE. 



Area and General Characters. The Galena limestone, the 

 upper formation of the Trenton group, occupies a narrow 

 strip of country, seldom exceeding twelve miles in width, 

 although it is fully one hundred and fifty -five miles long. It 

 is about two hundred and fifty feet thick in the vicinity of 

 Dubuque, but diminishes in thickness as it extends to the 

 northwestward, so that it does not probably exceed a hundred 

 feet where it crosses the northern boundary of the State. The 

 area it occupies is widest about the middle, but it becomes 

 very narrow at both ends; at the southeasterly end by 

 passing beneath the Upper silurian rocks, and appearing 

 only in the face of the bluffs which border the Mississippi 

 river, and at the northwesterly end by the thinning of the 



