UINTACRTNUS: ITS STEUCTUKE AND RELATIONS. 5 



fined to the outcrop. All of these layers, whatever their color, are chalk, 

 more or less pure, composed of the remains of Foraminifera. The impuri- 

 ties, according to Williston,"^ vary from less than two to ten per cent. 



An account of the microscopical investigation of the Niobrara Chalk, 

 and description of the organisms occurring in it, by Mr. C. E. McClurg, 

 may be found in the Report of the University Geological Survey of Kan- 

 sas, Vol. IV., p. 415 et seq. To the presence of the soft, muddy bottom 

 of a quiet, mediterranean sea, or lagoon, forming this deposit, is due the 

 excellent preservation of the fossil remains. Those Crinoids that were at 

 the time at the lowest part of the floating mass rested directly upon the 

 soft mud, and settled into it, in the position in which they happened to be. 

 They were thus perfectly embedded by the lower side in a fine matrix, 

 which would preserve them like a mould. The others piled on top of 

 them, and not having any such soft or plastic bed to receive and preserve 

 them, were crushed out of shape, disarticulated, and their calyx plates and 

 brachials were indiscriminately mixed up. These were cemented together 

 by pressure, forming the slab — a thin layer of limestone as we now find 

 it — with the Crinoids preserved only on its under side. These should all 

 be approximately perfect, — except for one fact hereafter explained, — and 

 if they could be taken up with about an inch of the chalky mud ad- 

 hering, and then have this very carefully removed, most of the specimens 

 on the lower surface would be quite perfect. In practice this is impossible, 

 for the only line on which the layers will separate with any regularity is at 

 the junction of the limestone slab with the chalk, and in many cases those 

 parts of the Crinoids which were deepest embedded in the matrix are 

 pulled off and left in the opposite surface. This almost always involves 

 the finer extremities of the arms, which are so delicate as to preclude 

 recovery out of the matrix. In some cases the parts adhering to the 

 chalk were, by great care and patience, cut out with a block of the matrix 

 and pasted upon the Crinoid from which they had been broken, and then 

 the block of matrix afterwards worked away ; but in the case of the fine 

 extremities of arms this could not be managed. Hence it is that we do not 

 ever get the arms preserved to their full length. But enough of them 

 was found to show nearly their full length, and to furnish complete 

 proof of what it must have been in life. The injuring of the specimens 

 by the pulling off of plates and arms has its compensations, however, for 



* Univ. Geol. Surv., Kansas, Vol. I., p. 239. 



