4 University of Texas Bulletin 



markings, such as I had never seen figured or described. 

 The rock was evidently a block from the Dakota sandstone. 

 Its smooth upper surface, which represented a bedding plane, 

 was covered with a thin coating of silt or fine clay which 

 adhered to the block. The markings were in this clay. 

 They were straight, shallow grooves from one-half to two 

 inches in length, and from one-sixteenth to one-eighth inch 

 in width. They were joined into patterns in which some 

 sprang out from the sides of others and again themselves 

 sent out other branches. Some crossed each other. I no- 

 ticed that there was a quite uniform angle of divergence in 

 these branches, and I was able to make out that this usual 

 angle was about sixty degrees. I also noted that the grooves 

 narrowed to sharp points. Somehow, immediately I con- 

 cluded that the cracks were the result of ice crystals, and 

 I at once saw the propriety of frozen water having existed 

 in an age during which deciduous trees began to appear. 

 This was theory. We have since that time learned to know 

 that cold climates far antedate the coming of the dicotyle- 

 dons. 



As I had no suitable photographic equipment, I took pains 

 to make accurate drawings of a part of the pattern as it 

 appeared on the rock. My original drawing is shown in 

 Plate I. A brief description of these markings was later 

 furnished in the Scientific American, of February 19, 1895. 



It took me some years to find any similar markings again. 

 In the early spring of 1903 I had occasion to make a visit 

 to Mexico, when I spent a half day in Ojinaga, which is a 

 little village south of Presidio, in Texas, on the Mexican side 

 of the river. Some sidewalks in this little village are built 

 of flags of limestone belonging to the Eagle Ford formation. 

 To my great delight I found some of these slabs having 

 precisely the same kind of markings that I had noted on the 

 sandstone in Dakota. Naturall}^ I attached some importance 

 to the fact that the Eagle Ford corresponds quite closely 

 in age to that of the Dakota sandstone. Both were maae 

 at about the beginning of the upper Cretaceous age. I 

 noticed here a considerable variation in the closeness of 

 the patterns of the markings. Occasionally they were found 

 as separate single lines, several inches removed from each 



