B. T. Hill — Cretaceous Formations of Mexico. 309 



The Lower Cretaceous Formation. 



The Mountain Limestone. — The most common and striking 

 material in the composition of the Mexican mountains is the 

 firm stratified limestone which, with the eruptive rocks, forms 

 the chief mass of all the Sierras. It constitutes the prevalent 

 sedimentary rock of nearly all the mountains and in the south- 

 ern States extends in intermittent patches from ocean to ocean. 

 Its strata produce the peculiar scenic effects of the mountain 

 topography and its products are the principal material of 

 the later Basin deposits. Lithologically the lime stone consists 

 of blue and grey, sometimes nearly black, beds of great hard- 

 ness and density, and indistinguishable lithologically from the 

 Paleozoic limestones of the Silurian and Carboniferous forma- 

 tions of the European and Appalachian regions to which they 

 have frequently been referred from this resemblance.* 



The strata vary in thickness from a few inches to a hundred 

 feet. They are so broken and distorted by the mountain 

 structure (fig. 1) and disconnected by the concealment of the 



Example of folding of Comanche limestone in Eastern Cordilleras. — Section 

 northwest of Monterey. 



basin deposits that it is impossible to give an idea of their total 

 thickness or to obtain a complete section of the series in any 

 one locality. In the eastern Sierra Madre in the valley of 

 Miquehuana where the base of the series was studied by Mr. 

 McGee and the writer, twenty thousand feet of the limestone 

 could be seen in succession in the continuous strata between 

 that point and the Hacienda El Carmen to the westward, all of 

 which was only the lower portion of the great formation. 

 This enormous thickness of the mountain limestone is further 

 indicated by the frequent sections of 5000 feet of portions of 

 the formations often visible in the tilted mountain fronts. 

 Even at the Rio Grande near Presidio del Norte the same lime- 

 stones as shown by White, attain a thickness of at least 4000 

 feet.-f- The persistent characteristics of this limestone with 



* The details cannot be given here of the innumerable references of the Cre- 

 taceous mountain limestone to Paleozoic age. Nearly always it was mistaken for 

 Silurian or Carboniferous by mining engineers, and early writers on the country. 

 With the exception of certain localities in Sonora given hj R. Remond, it is ex- 

 ceedingly doubtful if there are any limestones in Mexico of Paleozoic age, 



f This Journal, xxxviii, p. 440, 1 889. 



