J. E. Hyde — Desiccation Conglomerates. 407 



less rounded and subangular. Those less than an inch in great- 

 est diameter are usually less angular and do not show their flat 

 character so strikingly if at all. The larger blocks are exactly 

 of the type that would be formed on a mud-cracked surface 

 and the concave surfaces would not be found in a beach or 

 stream-rolled pebble except in fragments plucked from thinly 

 bedded strata which had not been worn to any extent. Fur- 

 thermore, at one point an area one and one-half feet long by 

 one foot wide was seen in which all the fragments remain in 

 the relative positions in which they were formed by the crack- 

 ing. These fragments are three-fourths of an inch to an inch 

 in thickness and one to two and one-half inches across. The 

 edges are fairly sharp and the cracks are filled with many 

 much smaller fragments. Occasionally, two little-worn pebbles 

 are seen entirely surrounded by broken fragments but still 

 holding their original positions with relation to each other. 

 Examples of these are shown in the outline sketches, which 

 were all made from a surface which otherwise showed no trace 

 of mud-cracking. The limestone composing all the pebbles is 

 indistinguishable from that of the underlying bed. 



The matrix in which the pebbles rest consists almost entirely 

 of small limestone fragments which, as just stated, might better 

 be termed a very coarse sand, throughout which are numerous 

 small fragments of teeth and bones of fish, many ostracod cara- 

 paces and shells of Spirorbis anthracosia Whitf. Angular 

 quartz sand grains are common although constituting only a 

 small percentage of the whole, the largest being from - 5 to "7 

 millimeters in diameter. A single rounded weathered frag- 

 ment of coal was also found on the surface.* The matrix is 

 usually about one-fourth or one-half an inch thick, but under 

 some of the pebbles it may thin away to a mere trace. It rests 

 on the underlying compact amorphous limestone without any 

 gradation into it although the two are so closely bound together 

 that they do not split apart. 



The surface of many of the pebbles, especially of the smaller 

 ones, appears to be extensively weathered ; this is notably so 

 when they are so turned as to expose a side which cuts across 

 the planes of sedimentation. The fine laminae, not readily 

 noticeable on a fresh fracture, have caused unequal weathering 



*This piece is of more than passing interest. It is only two and three-tenths 

 millimeters in diameter, is rounded and has the lustreless black etched sur- 

 face of a piece of coal which has been rolled along a stream bed. The 

 fracture is bright and smooth like that of fresh coal and the hardness is that 

 of coal. The bed from which it was derived must have passed through all 

 the stages of coal formation and must have been in the condition shown in 

 the fragment before it was eroded, for it is inconceivable that the piece 

 could have been formed in position where found. This would mean that 

 some coals at least had progressed in their formation to this stage before the 

 deposition of the Pittsburg coal. 



