76 



THE MUSEUM 



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Prairie Hens — Tympanuchus americanus. 



boulders that are found within the 

 borders of this strip. 



Scattered over its entire area are to 

 be found immense numbers of loose 

 stones, varying in size from minute 

 pebbles to huge boulders of many tons 

 weight. Although there are many 

 varieties which have been brought 

 from beyond the limits of the strip, 

 yet, by far, the greatest number of 

 boulders consist of the same material 

 as the underlying rock, viz: clay, slate, 

 blue limestone, granite and milky 

 quartz. Of these, perhaps the blue 

 limestone boulders are the most inter- 

 esting to study. They certainly pre- 

 sent the most difficult problems to 

 solve. 



In shape these boulders are nearly 

 always thin, flat and more or less cir- 

 cular. Ordinarily they are not over 

 eight feet in diameter and six or eight 



inches in thickness, but it is not un- 

 common to tind them much larger. 



The largest one that I have ever 

 seen measures a little over twenty-six 

 feet in its longest diameter. The 

 thickness bears some proportion to the 

 diameter, and this stone is not far 

 from fifteen inches thick. When ex- 

 posed to the action of the elements, 

 the stone disintegrates and forms a soft, 

 porous, gritty substance, of a dark 

 brown or black color, commonly 

 known as "rotten stone." Hence, it 

 is that small stones of this variety are 

 seldom found unless they have been 

 recently broken. 



A stone with a diameter of less than 

 ten inches or a thickness of less than 

 one inch is rare. The stone is easily 

 broken in the line of its longest dia- 

 meter; it breaks less rapidly in the 

 line of its shortest diameter; with 



