12 GENERAL OBSEEVATIONS. 



SUBCARBONIFEKOUS PERIOD. 



The physical conditions that prevailed during the Subcarboniferous 

 period over what is now the North American continent were quite various; 

 but the rocks of the period possess general distinctive characters in their 

 fossil fauna, by means of which they may be separated with little difficulty 

 from those of the next succeeding period. It is in the valley of the Missis- 

 sippi that they are most characteristically developed, and A^'here five distinct 

 formations, marking as many epochs, are found. The names of these for- 

 mations in the ascending order are the Kinderhook group, Burlington lime- 

 stone, Keokuk limestone. Saint Louis limestone, and Chester limestone. 

 Each formation has its own characteiistic fauna ; but through all of them a 

 few species are continuous, even into the strata of the next period, the Car- 

 boniferous. Taking these Subcarboniferous formations together as the stand- 

 ard for the group, it has been found more or less difficult to recognize their 

 respective equivalents among the rocks of the same period, either eastward 

 or southward from that region, or, at best, the order of succession is nowhere 

 so well shown as it is in the region referred to. In the Rocky Mountain region, 

 it has been found that there is, at many localities, a greater or less mingling 

 of Subcarboniferous with Carboniferous types ; but at quite a number of 

 localities, collections have been made that are regarded as distinctively Sub- 

 carboniferous. Only a very few of these collections, however, exhibit a fauna 

 clearly referable to any particular one of the formations of the period that 

 have just been named. Perhaps the most remarkable of the collections of 

 this character is the one made by Professor Bradley in Idaho, the fossils of 

 which Mr. Meek has found to be strikingly characteristic of the Saint Louis 

 limestone as developed in some parts of Indiana. 



The collections of the expedition contain fossils from only three locali- 

 ties that I have definitely referred to the Subcarboniferous period. These 

 localities are Mountain Spring, Old Mormon road, Nevada ; Ewell's Spring, 

 Arizona (upper horizon) ; and a place below Ophir City, Utah. The collec- 

 tion made at the first-named locality is the most characteristic and important 

 one of all, and is referred to the horizon of the Kinderhook formation, to 

 which horizon it is not improbable the otliers also belong. As the recogni- 

 tion of distinct epochs of the Subcarboniferous period in the Rocky Mountain 



