SlAY 3, 1907] 



SCIENCE 



711 



-when the Gulf of Mexico occupied this area. 

 They now occur as gray, blue and black 

 strata intensely hard, greatly altered, and in 

 places converted into white crystalline marble 

 and in others into a semi-schist. The indi- 

 vidual beds of limestone vary in proportions 

 of lime, silica and clay, and also slightly in 

 «olor and texture. 



These limestones have been greatly tilted 

 and deformed in the various processes of 

 mountain and continent building, so that in- 

 stead of lying horizontal as they were origi- 

 nally deposited, they are tilted at every con- 

 ceivable angle. They have also been folded 

 into close folds. These folds are the most 

 -complicated I have ever seen in the rocks of 

 this period, constituting what geologically may 

 he termed thnisted overthrows, recumbent 

 .anticlines, etc., of the alpine type of structure. 



When viewed from the plain from any direc- 

 tion, the mountain appears an elongated dome 

 ■with the strata dipping conformable with the 

 slopes in every direction from the central sum- 

 mit axis, so that as a whole it is quaquaversal 

 in its character. 



The western slopes of the mountain appar- 

 ently consist of uniform beds of strata dip- 

 ping towards the plain. On ascending and 

 -carefully studying the salients, it was found 

 that the strata were folded directly back of 

 one another in close compact recumbent folds, 

 and that the dip of the folds corresponds with 

 the ascent of the slope. Upon reaching the 

 shoulder below the summit of Picacho the 

 worn-off edges of the outer layers of the over- 

 thrown anticline project upward in great 

 vertical strata. 



The higher peaks and narrow connecting 

 summit ridges with the exception of Mount 

 Anticline, at the north end of the ranges, are 

 aU composed of the truncated vertical strata 

 of the overturn of the recumbent anticlines. 

 Not only are the rock sheets closely folded, but 

 the folded edges are buckled, or bent, so that 

 the buckling of the folds corresponds in its 

 sinuous course to the axis of the mountain. 

 The general strike of the old folds is north 30 

 «ast, north 50 east and north and south. 



Had it not been for my previous familiarity 



with every detail of these Cretaceous rocks, as 

 elsewhere studied, I doubt if the true struc- 

 ture would as yet have been ascertained. In 

 fact, it was not until after many days upon 

 the ground that I discovered, by the aid of 

 the layers of flint nodules embedded in the 

 limestone, how to trace the stratification beds 

 in these greatly disturbed and altered strata. 



Alteration. — In places near the acute 

 flexures, the limestones are completely meta- 

 morphosed into white crystalline marble, or 

 converted into schists — a fact which I have 

 previously noticed in Mexico at Guaynopita, 

 west of Chilpancingo and other localities. 

 This phenomenon is so frequent as to lead to 

 the inquiry if much of the pre-Gretaceous 

 plexus of Mexico has not lost its integrity 

 through the intense metamorphism of the 

 post-Cretaceous mountain movements. 



These limestones are also excessively frac- 

 tured by several systems of joints, faults, etc. 

 The bedding planes, which, when the strata are 

 vertical, are also exceedingly difficult for the 

 layman or even the geologist to distinguish, 

 from the faults and joints. 



In this mountain two distinct systems of 

 faulting are discernible. The first system 

 may be known as the north-south and north- 

 east system, the other as the northwest fault 

 system. The north 80° west faults are related 

 to the northwest system in age. 



The faulting and folding in north-south 

 and northeast directions is an older structural 

 feature than the other faults mentioned, 

 and is apparently barren of mineralization. 

 Faulting of this kind may be seen on the 

 northeast side of the Picacho and the east 

 side of Placeras and the west side of America. 



Many of the north 40° west faults occur in 

 parallel belts across the mountain range and 

 nearly all the mineral outcrops seem to be 

 closely associated with them. 



The movements along the northwest faults 

 has been lateral, displacing the older folds. 



No igneous rocks outcrop on the Almoloya 

 Mountains, or anywhere within a distance of 

 ten or twelve miles, so far as I could ascer- 

 tain. Fragments of rolled rhyolite occur in 

 the talus along the west base of the mountain, 

 and pieces of basalt are found in the Julietta 



