208 ILLINOIS ACADEMY OF SCIENCE 



the left bank has a steep wooded slope to one of the character- 

 istic curving bluffs. At one place this bluff was deeply under- 

 cut, as at the Sand Cave and Clarida Springs, and at many 

 other places where the millstone grit appears. In fact, a cave 

 had formed. But the dome of the cave fell in and most of it 

 was washed away, so that the water from the slope above now 

 falls over a new cliff further back and runs under a natural 

 bridge whose arch is 25 feet wide, 16 feet thick and whose un- 

 der surface is 24 feet from the bed below. The span is 150 feet 

 in the clear. On three different occasions, armed with the best 

 of cameras, but not with climbers and axes, we have tried to 

 photograph it successfully, but it is too big and too high up 

 the slope, with too many trees in front, to be readily photo- 

 graphed. 



There is abundant natural material in these few counties 

 to illustrate all the essential points in a high school course in 

 physical geography. Much valuable material is lying unused 

 because it has never been put into the books for those who are 

 not near it, and because it is not appreciated by those who do 

 live near it. 



THE PRESENT STATUS OF THE DOLOMITE 

 PROBLEM 



ARSTRACT 



By Francis M. Van Tuyl, Colorado School of Mines 



There is convincing evidence that the great majority if not 

 all of the important sedimentary dolomites have resulted from 

 the replacement of limestone through the agency of solutions 

 bearing magnesia. Most of the data favors the view that the 

 alteration took place before the limestones emerged from the 

 sea. But the conditions under which this was effected are not 

 certainly known. 



(Complete paper published in Science, N. S., Vol. XLIV, 

 No. 1141. November 1, 1916.) 



