3o8 The American Geologist. ^^>'' ^^^*- 
ified fragments will dip toward the centre of the mountain (in- 
trorse), if, on the contrary, the core be of metamorphic material 
the strata will dip from the centre or axis (extrorsej. This 
law seems of quite general application. In the Ladrones, Mag- 
dalenas, Sandias, Manzanas, San Andres, Oscuros, Caballos, 
Fra Cristobal, and numerous others, the dip is away from the 
granitic core. In the Limitar, Socorro, Black mountains and 
many more, where the core is igneous, the strata dip toward the 
theatre of activity and the evidences of interaction are very in- 
teresting. In volume I of the Bulletins of the University of 
New Mexico the writer has given brief description of both 
the Limitar and Socorro mountains and the relation of the sed- 
imentaries to the later intrusives is sufficiently discussed for 
the present purpose. In that paper attention is called to the 
curious fragmental andesyte tuff above the limestone from 
which it derives its calcite magma. Toward the north end of 
the ranges, at a point not visited at the time the paper was writ- 
ten, there is an area of explosive volcanic activity of the rhyol- 
yte period which affords a very interesting illustration of the 
method of formation of talus conglomerates. Two fork-like 
rhyolyte ridges, miiting with the main peak of the Limitar at 
their base or point of union, embrace an area of over a mile 
square filled with the evidences of explosive flows. The material 
is all rhyolitic, but it has been poured into water and suffered 
some rounding afterwards, and then has been compacted into 
irregular conglomerate, breccia (near the rhyolyte itself) and 
tuffas of all sizes, the materials growing smaller as one leaves 
the focus. The dip is high and away from the focus, forming 
an arch to the east, northeast, and north. Surrounding this 
area is what has been regarded as late Tertiary in well-stratified 
beds composed of similar materials, but with some admixture 
of other elements from the range. The dip is confused and 
the materials at a distance from the mountain grow finer and 
red in color. They seem to belong to the same series as the 
(Pliocene) Tertiary sandstones so prominent on the east side 
of the Rio Grande at this place. These beds disappear under 
what we have called the clinoplains of the Pleistocene age. 
In the papers on the Socorro and Limitar mountains we 
considered that the Rio Grande occupies a great monocline 
on the east side of the river. The data in substantiation of this 
