216 GEOLOGY. 



311. Sub-carboniferous Period. That first part of the 

 Carboniferous age in which the limestone floor was laid 

 down for the coal-measures to rest upon is called by ge- 

 ologists the sub-carboniferous period. The prefix sub 

 being the Latin for under, you see the propriety of the 

 term. This floor was laid down over a very large area 

 in this country. The same thing is found in other coun- 

 tries where coal was made in this age. The thickness 

 of this formation varies much in different localities, in 

 some amounting to more than 6000 feet. As this lime- 

 stone was made from the remains of marine animals, 

 crinoids, corals, etc., long ages must have been occupied 

 in constructing this foundation of the coral-fields. The 

 strata of the proper carboniferous rocks, the rocks of the 

 coal-measures, are conformable with the sub-carbonifer- 

 ous. 



312. Sub-carboniferous Period in this Country. The 

 circumstances under which the sub-carboniferous lime- 

 stone was formed in the United States have been admi- 

 rably brought out by Professor Dana, and I will attempt 

 briefly to make the principal of them clear to you. In 

 the Niagara period of the Silurian age there was a for- 

 mation of limestone over a large portion of the interior 

 of the United States. This was made by the deposition 

 of the remains of marine animals. While this was going 

 on all the interior part of the country was a sea, not very 

 deep, but of sufficient depth for these animals. This 

 sea might be considered as an extension upward of the 

 present Gulf of Mexico. It was cut off from direct com- 

 munication with the Atlantic Ocean by a low barrier of 

 land stretching from the north downward, which had 

 been made to emerge in order to shut in this great inte- 

 rior sea. A similar condition of things existed in the 

 Devonian age, when, in the Upper Helderberg period, 

 the corals were so busy in laying down limestone 

 ( 293). A similar arrangement existed also in the sub- 

 carboniferous period, except that the interior sea was 



