222 



SCIENCE. 



[N. S. Vol. XVII. No. 423. 



The Saddle-hack Topography of the Boone 



Chert Region, Arkansas: A. H. Purdue, 



University of Arkansas. 



The rocks exposed in northern Arkansas 

 are those from the Silurian to the Upper 

 Carboniferous inclusive. The position of 

 the rocks is essentially horizontal. In the 

 western part of the region north of the 

 Boston Mountains, the prevailing surface 

 rock is the Boone chert; but in central 

 northern Arkansas erosion has continued 

 far below the Boone chert, exposing the 

 Silurian rocks, except where there are 

 monadnocks. These monadnoeks are of 

 necessity greatly dissected by small moun- 

 tain streams which are cutting their way 

 headward into them. The spurs between 

 these streams are frequently surmounted 

 by one or more knobs, which produce the 

 saddle-back topography so common in the 

 region. These knobs are always capped 

 by fragmentary Boone chert, which in 

 some cases is partly water-worn. That 

 the water-worn material is not due to sub- 

 marine action is proved by the fact that 

 it is found only locally. It follows that 

 it must be of stream origin. 



The knobs owe their existence to the 

 formation of alluvial cones by former 

 streams at the time when the stream beds 

 were on the level of the present knobs. 

 The cones of the fragmentary chert ob- 

 structed the streams, causing them to 

 shift laterally, and at the same time pro- 

 tected the subjacent rocks from erosion. 

 If conditions were favorable, two or more 

 cones Avere formed by the same stream at 

 different stages in its history, a knob re- 

 sulting from each cone, and a series of 

 knobs indicating the former course of the 

 stream. The paucity of the water-worn 

 material in these old cones is explained by 

 the streams that formed them having been 

 short and of a torrential nature. The 

 present drainage of the region is like that 

 at the time the Boone chert was the pre- 



vailing surface rock only as regards the 

 master streams; and the change is due in 

 a large measure to the shifting of the 

 streams brought about by their own ob- 

 structions. 



Scientific Belief Maps: Gborge Carroll* 



Curtis, Boston, Mass. 



For years past the scientific bureaus of 

 the United States have deemed it advisable 

 to construct relief maps. The Paris Ex- 

 position of 1900 afforded a just interna- 

 tional comparison; and it was found that 

 America was exhibiting work which fell 

 far behind that of some of the European 

 countries, because of the lack of scientific 

 methods. 



A perfect topographic relief map is a 

 perfect miniature or model of nature ; and, 

 unless natural laws and principles are em^, 

 ployed in its construction, no progress be- 

 yond the old relief map will be attained. 

 Truthful topographic modeling is an exact 

 art requiring accurate, rational and sys- 

 tematic methods throughout the gathering 

 of data upon the field, and the application 

 and reproduction of the facts of nature 

 in other dimensions. Modern scientific 

 inventions, including contour maps and 

 dry-plate photography, are aiding in bring- 

 ing this work toward perfection and into 

 recognition. 



Lunar Calderas: E. Hayes, Wellesley Col- 

 lege. 



Every topographic feature of the moon 

 is invested with mystery and difficulty. 

 Some forms, however, invite study and 

 discussion, because of their likeness to cer- 

 tain earth forms. Among these are the 

 so-called 'ring-plains.' They consist of a 

 circular wall, composed often of lofty 

 mountains enclosing an approximately hor- 

 izontal floor which is generally broken by 

 a central cone. The prevailing theories of 

 their formation are untenable, for both 

 dynamic and topographic reasons. On 



