316 PROCEEDINGS OF THE ACADEMY OF [1893. 



age and which Dr. Comstock 5 considers true Archean. The extent 

 of this area is at present limited and there is no means of determin- 

 ing its former boundaries. The strata are highly metamorphosed, 

 in part by intrusions of Plutonic rocks. About this core are Cambrian 

 and Silurian strata, according to Dr. Comstock folded and faulted in 

 an extremely complicated manner. These older rocks obtained most 

 of their complication and metamorphism before the beginning of 

 the Carboniferous period, for the strata of the latter age rest with 

 marked unconformity on the northern slopes of the Silurian hills. 



What was the limit of this Pre-Carboniferous land we have no 

 exact means of determining, but that the present exposure by no 

 means expresses the original extent of outcrop seems certain from 

 the vast quantities of material in the Carboniferous and later strata. 

 Moreover the conglomeratic and sandy nature of the lower Carbon- 

 iferous deposits, along their eastern margin, indicates very strongly 

 that beneath the Cretaceous not far distant from this, the older 

 land exists. I believe it more than possible that this buried land 

 extended to the areas of Indian Territory and Arkansas, thus con- 

 necting them with the central core of Texas. 



Be this as it may, there was, in one part at least, a land area in 

 immediate Pre-Caiboniferous time. This land was mountainous in 

 type, so far as structure is concerned, and had practically its 

 present degree of metamorphism. 6 Whether the land was topo- 

 graphically mountainous or not I cannot say. It certainly was not 

 so in the immediate neighborhood of the Carboniferous, for the 

 valleys and hills which were buried beneath the Carboniferous strata 

 were low and well rounded, being mature in type, much more mature 

 than the present topography of the same district, and quite closely 

 like that of the less hilly parts of New England. For this reason 

 I assume that all the land in the neighborhood had undergone long 

 continued denudation and was mature in geographic form. It was 

 a mountainous area worn to its roots in Pre-Carboniferous time. 



(b) Carboniferous Modification of the older Pakeozoic Land. — The 

 next event in the history is the oncoming of the Carboniferous de- 

 pression and period of sedimentation, a period introduced apparently 

 in Sub-Carboniferous times and continuing with slight interruption 

 to the close of the Permian. The evidence of a Sub-Carboniferous 



5 1st Ann. Rept., Texas Geol. Survey, 1889, pp. 255-82. 



6 This fact is proved by the exigence of nimble and flint pebbles in ihe 

 Carboniferous, with the same stiucture that the mateiials at present have in the 

 neighboring Silurian. 



