82 Bulletin of Laboratories of Denison University voi. xi. 
trusive flows. It is also quite the rule in this as in other similar 
regions that the massive flows should catch up fragments of the 
adjacent or perforated country rock and incorporate them more 
or less completely in the pasty or fluid magma. It then, of 
course, follows that these fragments are themselves more or less 
altered while, if the heat was sufficient, the magma is likewise 
modified by the introduction into it, at various stages of cooling, 
foreign ingredients. In fact, nature has here performed for us 
some of the most difficult experiments feebly attempted in mod- 
ern laboratories of synthetic geology and the study of the alter- 
ation phenomena both chemical and physical occurring in the 
rock fragments and in the “symmorphic” result of the integra- 
tion of the various disparite elements by secondary fusion 
promises more for this branch of geology than any method 
available and yet, curiously enough seems hardly at all worked.^ 
We may picture to ourselves a time when the Cretaceous 
strata lay in practically undisturbed possession of the region. 
The time was long past when coal swamps occupied the great 
basins to the west and south-east and gradual elevations alter- 
nating for many years with slight depressions had afforded the 
conditions necessary to the formation of beds of gypsum and 
salt.^ We may suppose that a long period of elevation had 
passed over the region without leaving any record except in ex- 
tensive denudation and an occasional local deposit. There were 
doubtless secular contractions of the crust but at last a time 
came when the intrinsic strains overcame the rigidity of the 
crust and great orographic lines of weakness developed in ap- 
proximately prrallel lines extending north and south over the 
whole area of the southern Rockies. The present course of 
the Rio Grande in central New Mexico occupies what may once 
have been the axis of such a fold, while the intersection of the 
western declevity with the present general level is marked by 
^ Cf. Endlich, On the erupted rocks of Colorado. Hayden Survey Report 
for 1876. 
2 Salt springs occur only a few miles north-west of the Limitars and cause 
the waters of the Rio Salado to be decidedly salt to the taste. 
