I04 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



In the neighborhood of Alpena, Michigan, occur two forms equally 

 prevalent. One retains the relative proportions of the Davenport 

 crinoid, but ditfers from it in scarcely ever attaining one-third of its 

 size. The other is basin-shaped, with broader base than the preceding, 

 steep, low walls, and correspondingly low dome. This seems to have 

 had no representative in the Davenport locahty. Such variations, 

 while of interest as exhibiting the result of differing environments and 

 geologic time, do not warrant s])ecific distinctions and description. 



The condition and cliaraiter of the deposits in the two localities 

 are not without interest. In the quarries near Davenport, at least 

 in those portions to which the crinoids are restricted, we find thick, 

 heavy beds of rough, compact hmestone, without any partings of shale. 

 While fragments of crinoids abound, yet in most instances they are 

 partially imbedded in the rock, and cannot be extricated without dan- 

 ger of breakage. Everything of present environment suggests condi- 

 tions most unfavorable to their perfect preservation. On the other 

 hand, in Alpena, Michigan, we find a series of thin bedded limestones, 

 and interlaminated beds of soft argillo-calcareous shales, in both of 

 which crinoids are found, and from which they may for the most part 

 be readily detached. The surroundings indicate a condition of things 

 favorable to their existence when living or their preservation when 

 dead. 



The two localities are further distinguished by their relative place i?i 

 the series of which they form subordinate parts. Near Davenport these 

 crinoid-bearing beds lie beneath the shales and shaly hmestones that 

 go to make up the series — at the very base of the Hamilton group, if 

 not below it. It is claimed by Prof. Rominger, of the Geological Sur- 

 vey of Michigan, that the crinoid-bearing rocks near Alpena head the 

 series — crown the very summit of the Hamilton.* 



♦See Geolog-y of Michigf.in, Vol. III., Chap. VI. 



