854 LYLE YOORNAE NOR AGIZO FOGWé 
to modern nomenclature the rocks would now be called porphyrites, 
and the recent study with better thin sections confirms the conclusions 
of Dutton that the specimens collected from laccolites and dikes indi- 
cate no differences of composition or structure corresponding either to 
geographical or geological distribution, or to the size or form of the 
intruded masses. 
The West Elk Mountains are described and the numerous instances 
of laccolitic mountain masses noted. Some of these masses were 
described by Holmes and Peale in reports of the Hayden Survey for 
1873 and 1874. They have been more recently studied by Cross. The 
individual mountain masses of this group consist of homogeneous 
igneous rock, which in a number of cases is clearly shown to be intrusive 
within sedimentary strata, and is accompanied by sheets and dikes of 
the same eruptive rock. The character of the rock is very nearly the 
same as that of the Henry Mountain laccolites. The microstructures 
of the rocks of the different masses are practically the same, indi- 
cating that even the highest bodies solidified under a great load. The 
strata in which they have been intruded is Cretaceous, and the age of 
the intrusion is Tertiary. 
The isolated mountain groups of San Miguel, La Plata, Carriso, 
El Late, Abajo and La Sal, which were studied by Holmes and Peale, 
are reviewed and their structure noted as that of laccolitic intrusions 
within nearly horizontal strata. The rocks in all cases are of the same 
general type of porphyrites. Intrusions within more or less disturbed 
strata of older age and also within the crystalline schists, which have 
been studied by Cross, are described. They occur in the Mosquito 
Range and the Ten Mile District. Here the igneous bodies form 
sheets and dikes, the rocks having the same general structural charac- 
ters as in those previously described, but having a somewhat wider 
range of composition. Other occurrences of similar intrusions within 
the ranges of the Rocky Mountains are mentioned. In conclusion 
Cross points out the fact that while the rocks forming these intrusive 
bodies show considerable variation in composition, the great majority of 
them belong to one well marked structural type, that of porphyry ; and 
that this is plainly the result of the similarity of conditions of consoli- 
dation upon magmas which are much alike in their controlling ele- 
ments. He adds that there is every reason to suppose that other erup- 
tive provinces may be characterized by intrusive masses of other rock 
types. Ifthe magma of another region were very different from that 
so common in the region described, another structural type might 
