710 



SCIENCE 



[N. S. Vol. XXV. No. 644 



per day. Meat is rarely eaten by any but 

 well-to-do Neapolitans and the main sources 

 of proteids are bread, spaghetti and beans 

 (Phaseolus) . It seems to me that in the lack 

 of recuperative power shown by the Neapoli- 

 tan workmen there is an excellent example of 

 the danger of minimizing the reserve fund 

 of proteids in the system, as suggested by Dr. 

 Meltzer. 



Joseph T. Bergen 

 Cambridge, Mass., 

 April 12, 1907 



SPECIAL ARTICLES 



GEOLOGY OF THE SIERRA ALMOLOTA, WITH NOTES 



ON THE TECTONIC HISTORY OF THE 



MEXICAN PLATEAU^ 



The Sierra Almoloya is situated in the 

 southern part of the state of Chihuahua, 

 about 25 miles west of Jimenez, and midway 

 between the latter town and Parral. 



This sierra is one of the numerous isolated 

 mountain blocks of northern Mexico, like 

 Santa Eosalia, Naica, Santa Eulalia and 

 others, which rise in solitude from the vast 

 area of surrounding arid plains constituting 

 the great Chihuahua province, between eastern 

 and western sierras, of the Mexican Plateau 

 portion of the northern cofdilleran region. 



The mountain is a long and narrow range, 

 about ten miles in length, extending in a 

 northeast-southwest direction and averaging 

 less than two miles in width. It is surrounded 

 on every side by a lower area of sloping plain 

 which has an altitude of about 5,000 feet at 

 the mountain base. The total altitude of the 

 mountain above the plain is fifteen hundred 

 feet, the peaks rising to an altitude of 6,500 

 feet above the sea, as far as could be deter- 

 mined by a careful aneroid study. To the 

 south of the range are several conspicuous 

 outliers of lower altitude as shown upon the 

 map. 



The range is dominated by a narrow axial 

 summit ridge, following the northeast-south- 

 west trend of the mountains. Numerous nar- 

 row tongue-like salients radiate from the ridge 

 to the plain, and separate by deep arroyos 



'Read before the Geological Section of the New 

 York Academy of Sciences, April 1, 1907. 



cutting back to the ridge (' capturing ' it) and 

 their valleys making great cirques, or amphi- 

 theaters between the salients. 



Arising from the central ridge are several 

 conspicuous summit peaks. The highest of 

 these, attains an altitude of about 6,500 feet. 



Like the other limestone mountains of the 

 Chihuahua province, this sierra reveals the 

 ancient wrinkled and folded structure of the 

 plateau prior to when it was buried in vast 

 beds of rhyolitic and andesitic volcanic ejecta 

 which once covered this whole portion of 

 Mexico, and which is still preserved in the 

 western Sierra Madre, and like the other 

 ranges mentioned the mountain represents the 

 resistance and survival of the hardest, in the 

 destructive atmospheric erosion and degrada- 

 tion of a once higher surface of the great 

 western plateau. 



The Sierra Almoloya is, therefore, a de- 

 structional or decadent form of a mountain, 

 representing a remnant of the former extent 

 of the rock material upward and laterally 

 which fact is not only testified in the degraded 

 shape of the mountain configuration itself, 

 but by the vast quantities of talus and debris 

 in process of forming on its surface and now 

 filling the surrounding deserts. 



Every detail of its relief such as its axial 

 direction, the character of its slopes, the 

 course of its lateral arroyos and other features 

 are conformed to the arrangement or structure 

 of the rock material composing the mountain, 

 such as the lines of stratification, faults and 

 folds, etc., to be later described. 



The exposed rocks composing the sierra con- 

 sist almost entirely of limestones of varying 

 degrees of purity constituting the main mass 

 and country rock of the mountain. Second- 

 arily these are mineral ores and exceptional 

 fragments of igneous rocks, the latter not 

 found in place of origin. 



The Limestones. — The mountain mass is 

 composed of stratified limestones of the 

 Comanche series of Lower Cretaceous age 

 largely and mostly of the particular formation 

 known as the Edwards limestone. 



These limestones originated as sea muds in 

 the form of chalk and chalk marls, accom- 

 panied by horizons of siliceous flint nodules, 



