56 IOWA ACADEMY OF SCIENCES. 



EVIDENCES OF DISTURBANCE DURING THE DEPOSITION OF THE 

 BURLINGTON LIMESTONES. 



BY F. M. FULTZ. 



In a general way the lithological characters of the Burlington limestones, 

 including both the lower and upper divisions, are the same. It is true that some 

 layers are more compact than others, some more massive and a few are even 

 crystalline enough in texture to imperfectly resemble marble, yet they all owe 

 their origin to the same source. The material comprising them is almost wholly 

 crinoidal. To such an extent is this true, that, with the exception of a very few 

 layers, it is scarcely possible to find a cubic inch of rock that does not show its 

 crinoidal origin. There are a few layers of shales, clays, etc., but for the most 

 part they are quite thin and form but a very small part of the whole. However, 

 they are deserving of some attention and I shall take occasion to refer to at least one 

 or two of them specifically. 



What I will endeavor to point out in this paper is, that during the deposition 

 of these limestones, there were some periods of disturbance. The evidences of 

 such disturbance are: (I.) The more or less abrupt changes in fossil forms. (2.) 

 Change in lithological characters. {?>.) Erosion and unconformability. I wish 

 to speak more particularly about erosion, but will first say a few things about the 

 change in fossil forms. 



I have already mentioned that the prevailing life was crinoidal. Not counting 

 synonyms there are probably between 350 and 400 species of crinoids found in the 

 Burlington limestones. The greatest number occurring in any one layer is not 

 more than one-fourth of the whole; usually much less than that. Besides, many 

 of these species do not lap over each other and there are several breaks where not 

 a single species bridges over the change from one stratum to the next higher, 

 without some difference in form. So universal a change in fossil forms would 

 indicate a sudden change in climatic conditions, and since in each succeeding 

 stratum there seems to be no diminution, either in number of species or individuals 

 — the genera remaining nearly the same and the species closely allied to former 

 existing ones — there must have been a comparatively early return to the former 

 conditions. Of course, while all life may have been extinguished at one point, no 

 doubt it flourished in full vigor at no very great distance away, and as soon as the 

 conditions again became favorable it once more occupied its old ground. If the 

 period of interruption was short, or the area of disaster not too widely spread, the 

 new forms of life would not differ much from the old. But if the area of disaster 

 was extensive or the period of interruption prolonged, then the result would show 

 the extinction of species and the beginning of new ones. As a rule species do not 

 gradually die out, they are killed off. At least this is the apparent fact if the 



