76 CHARLES KEYES 



prevail. Over such a country geological boundaries are not easily 

 traced; and determination of the original areal distribution of the 

 various terranes is beset with exceptional difficulties. 



In most other parts of the world the local stratigraphic succession 

 and general mapping of rock tracts are based chiefly on the rock 

 exposures bordering the valleys of incised streams. These outcrops 

 as a rule lie below the general upland level of the country. In the 

 New Mexican field, the rock sections lie principally above the 

 general plains surface. Correlative determinations of outcrops 

 are thus exactly the reverse of what they usually are. 



Disposition of the sections, a mile or more high in many 

 instances, is that of a myriad of drill-cores set upon a board. 

 Spaces between sections are voids in nature as in model. Under 

 ordinary circumstances these intersectional intervals are filled 

 up by the rock masses of the interstream areas. In order properly 

 to visualize the geological formations of the tableland the various 

 sections have to be connected and projected on a common plane. 

 Such a ground plan is very different from a normal geological map. 

 Yet it is the only kind of a diagram that satisfactorily depicts the 

 larger relationships of the geological formations. For the region 

 under consideration such a projection is represented in the accom- 

 panying cut (Fig. i), the base plane selected being the ancient 

 peneplain lying at the base of the great Pennsylvanian limestone 

 plate. 



The fact that at the southern extremity of the Rockies the Penn- 

 sylvanian limestones everywhere rest directly on pre-Cambrian 

 schists long led to the inference that a region of continental pro- 

 portions had been a land area during the greater part of Paleozoic 

 times. Such apparently was not the case. When, a decade ago,^ 

 the geological formations of New Mexico were briefly described in 

 a systematic way, a circumstance that was expressly pointed out 

 was that while over all the northern half of the state there were no 

 Paleozoics below the Pennsylvanian limestones, it did not preclude 

 the existence of some, or even all, of the early periodic sections else- 

 where. In fact, in southern New Mexico isolated sections of these 



^Report of Governor of New Mexico to Secretary of Interior for ic)0j, pp. 337-41, 

 1904. 



