Art. V.] Herrick, Geology of New Mexico. 99 
geological record shows, in pretertiary time by the series of 
andesite eruptions and have continued through a trachytic and 
rhyolitic period and in post-tertiary time have been supplement- 
ed by the basaltic flows now so prominent in the topography of 
parts of this district. The Sandia range is separated from the 
continuation of the uplift to the south by Tijeras canon, itself 
marking, as we shall see, a transverse axis of disturbance. 
The length of the range is about 20 miles but the deflection of 
the valley of the Rio Grand^to the eastward in the northern 
part of our sheet causes the western escarpment to bear off in 
that direction and, this being in the direction of dip the height 
falls off rapidly in this part toward the north. The crest of 
the range is everywhere a band of Carboniferous limestone 400 
to 600 feet thick. The maximum thickness is not preserved, 
so that the upper horizons of this formation are better seen in 
the nearly horizontal beds to the east. 
The dip being from eight to twelve degrees to the south- 
east those peaks lying most to the westward are, of course, 
highest. The age of the limestone is abundantly attested by 
the fossils, while below them is a vast series of metamorphics 
the age of which it is impossible to more than conjecture. 
These metamorphics are of two groups which are distinguished 
by their lithological character and conditions of formation 
though they may be practically contemporaneous. In general, 
the limestone is underlaid by a mass of quartzite of varying 
thickness. This, where the metamorphism has been slight, is 
obviously of a sedimentary nature having conglomeritic phases. 
In other places this quartzite has been perfectly transformed 
into a granite or gneiss. It thus happens that in many places 
the limestone reposes on a crystalline granitic rock. In other 
places the granite is more than fifty feet below the base of the 
limestone. The second series of metamorphics is found at cer- 
tain foci of uplift. One such focus is seen in the foot hills to 
the west of Sandia peak, the high mountain at the north end of 
the ridge, another just at the mouth of Coyote Canon at the 
east end of the transverse axis of uplift which forms Tijeras 
Canon. Another and more important one occurs further south 
