MEXICAN BOUNDARY LINE. 125 



not such as usually accompany the production of coal. In the present instance, the ocean, 

 depositing the great limestone formations previous to the coal period, occupied to a great extent 

 the present area of the coal measures which succeeded. Land plants in excessive growth, 

 estuary or shallow water shells attend the production of the coal and its associated strata. We 

 begin thus to comprehend the truth that, during the period of the great coal formations of the 

 Appalachian, and the Mississippi valley, there existed a broad ocean at the far west and south- 

 west, in which these calcareous deposits were in course of formation ; that during the oscillations 

 which we know to have occurred throughout the coal period, tliere was a time when the whole area 

 became depressed so as to allow the waters of the southwestern ocean to flow over all the coal- 

 measure area, or, at least, as far northward and eastward as the northeastern part of Ohio ; and 

 from hence is derived the limestone under consideration. The calcareous bands deposited along 

 the northeastern margin of this ocean, we now find interstratified with seams of coal, and beds 

 of shale and sandstone containing land plants ; while, as the waters deepened towards the south- 

 west, the formation exhibits the differences of character which we would necessarily expect to 

 find in an ocean deposit. 



The evidence of the existence of this ocean in the far west and southwest, during the coal 

 period, amounts to almost a proof that the conditions of that area, which now constitutes a part 

 of the continent, were never such as to admit of the production of coal plants, and the deposit of 

 such materials as make up the coal measures, at least during the latter part of the coal period. 

 In regard to the earlier part of that period, or the time in which the lower coal measures were 

 formed, we have not, as I conceive, at present, the means of fully deciding what were the con- 

 ditions in the central and southwestern part of our continent. 



