Keyes — Significance of Certain Unconformities. 297 



fortuities have been found, and the positions of these are indi- 

 cated in the accompanying table of the major geological for- 

 mations of New Mexico. For present consideration the chief 

 value of the section lies in the surprising number of great 

 unconformity horizons and in the presence of extensive for- 

 mations which are represented by gaps in the rock-succession 

 farther north in the Rocky Mountain region. 



Since the time-value of the unconformity plane has been 

 seldom measured quantitatively in sediments, and since every 

 local geological section has an erosional as well as depositional 

 record, the former is not measurable in the locality in which 

 it is found, but finds expression in sediments in other and 

 neighboring localities. One of the most remarkable instances 

 of exact equivalency of this kind is found in the unconformity 

 plane in the Upper Mississippi valley at the base of the pro- 

 ductive Coal-measures. The depositional equivalent of this 

 erosion plane is believed to be found on the south side of the 

 Ozark dome in Arkansas, where the measurement is more than 

 18,000 feet. Southward beyond the end of the Rocky Moun- 

 tains in New Mexico and the adjoining states conditions exist 

 very similar to those in the Ozark region. Depositional equiv- 

 alents of some of the gaps farther north are here found. 

 Unconformities are also displayed in a way that enables the 

 geological history to be made out with very much greater 

 clearness than anywhere else. In this southern region, where 

 mountain-making movements have not been so great as further 

 northward, there has been recognized, as already noted, no less 

 than a score of great unconformities, beside many of minor 

 importance. The general geological section of this region, 

 together with the horizons of unconformity, are given below. 



Out of the 25 rock series which have been defined, there 

 are only two (and possibly two other) exceptions in which 

 these major formations are not separated by great planes of 

 unconformity representing profound erosion intervals. The 

 time values of these various gaps in sedimentation no doubt 

 differ very much among themselves ; but they are all great. 

 For instance, in central New Mexico, the Cimarronian Red 

 Beds rest directly upon the Maderan limestones, with marked 

 unconformable relationships. In southern New Mexico this 

 interval is partially occupied by no less than 2,500 feet of sand 

 stones and limestones. The relationships are indicated by dia 

 gram (fig. 1). 



Little is known as yet of the unconformities of the Proter- 

 ozoic within the boundaries of New Mexico. The presence of 

 a pre-Cambrian clastic sequence has only been very recently 

 recognized with certainty* and differentiated from the Arche- 

 ozoic crystallines. There appear to have been several ero- 



*Eng. and Mining Jour., vol. lxxvi, p. 967, 1903. 



