l6 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



development of animal life. In nearly all the exposures observed it 

 contains few fossils." * Professor Hall had access only to the expos- 

 ures above the city. Since that time quarries have been opened below. 

 The general character of the rock, to some extent, still prevails. That 

 it is unfavorable to the preservation of fossil remains is apparent from 

 the condition in which they are found. It holds them firmly, and gives 

 them up reluctantly. Ground into small fragments before the deposi- 

 tion of the rock, they are compacted into its substance. If large frag- 

 ments occur, they are generally broken or crushed. Even where a 

 somewhat perfect external form appears, in the majority of cases the 

 finer markings and the delicate organs, on the preservation of which 

 identification depends, are obliterated beyond all hope of restoration. 

 As a general rule, the rarer the fossil the more damaged is its condi- 

 tion. Of the genus Stereocrinus, one of our Crinoids, of which scat- 

 tered fragments had been found from time to time, it was not till 

 after six or seven years of exploration that a specimen was found in a 

 sufficiently good condition to warrant description and illustration. In 

 a single instance a reef is formed, abounding in fossils on its upper sur- 

 face. The most noticeable fact is nol the abundance of differing spe- 

 cies, but the recurrence of so many individuals of the same species. 

 While they exist in numbers partially weathered, it is to the exclusion 

 of every other form. At the same time, it is so difficult to extricate 

 the individual from the mass, that it would be impossible to find a sin- 

 gle well-preserved specimen in any cabinet in the city. From such a 

 character of the rock, we deduce that no great number of species may 

 be expected. 



As a second consideration, the limited exposures of the rock are 

 against it. In this respect, geologists at a distance, who have never 

 visited us, find it difficult to realize our situation. In proportion as 

 the exposures of a rock are multiplied, extended, as they often are, 

 along a river or some other natural exposure, or cropping out at inter 

 vals throughout a State, in that proportion we look for abundance of 

 species. We emphasize this fact. Our whole Corniferous limestone 

 in its fossiliferous portion comprises simply two or three slight expos- 

 ures above the city — now almost obliterated — a slight show between 

 Rock Island and Moline, also at Milan, and two or three small, insig- 

 nificant quarries below the city. This is the whole extent of the fossil- 

 bearing limestone as exposed in our vicinity. We are told that out of 

 this have come two hundred and forty-six species of fossils. 



♦ Geology <>f Iowa, Vol. I., \t. S4. 



