PUBLICATIONS. 855 
result from the same conditions of cooling. He points out the fact 
that the rocks of the plateau groups and those of the mountain area 
of Colorado differ in the relative abundance of alkalies, those of the 
latter area being higher in potash. These observations apply to all 
igneous rocks whether in the form of laccolites or not, and the rocks 
of the Yellowstone Park, whose analyses are placed in a table for com- 
parison with those of the rocks Cross has described, are not laccolitic, 
but dike-like intrusions. Since Cross wrote this paper Pirsson and 
Weed explored the Highwood Mountains and published the descrip- 
tion of the laccolite of Square Butte, whose rock is granular syenite 
and shonkinite, very different in structure and composition from the 
porphyries described by Cross. 
In discussing the mineral composition of the rocks Cross lays stress 
on the fact that in some cases large orthoclase phenocrysts have clearly 
been crystallized after other constituents which do not appear as phe- 
nocrysts. He finds no evidence that any of these eruptive masses 
“‘absorbed”’ sedimentary masses. ‘There is in fact almost no metamor- 
phism of sedimentary rocks along the contact with these igneous bodies. 
Owing to the great range in geological distribution and in size and 
form of masses of practically identical rocks, Cross concludes that the 
conditions of cooling or consolidation were almost the same through- 
out a wide (deep) zone, and that pressure as a function of depth has 
had very little influence within the limits represented. The rate of 
cooling must have been essentially the same for all masses, indicating 
that below a certain depth there is a zone in the earth’s crust within 
which conditions of cooling are practically uniform. The upper and 
lower limits of this zone are not known, but it would seem as if a 
depth of several thousand feet must be necessary to secure a tempera- 
ture so high and a rate of cooling so slow that the chilling effect upon 
an intruded magma should be no greater than that at a depth of 20,000 
feet. We do not find that Cross considers the consequences of mag- 
mas having quite different temperatures when they reach the place of 
laccolitic intrusion, or that he discusses possible differences of temper- 
ature of the enclosing rocks due to local causes. 
In describing the structure of laccolitic rocks, Cross discusses the 
terms porphyritic and granular, and expresses the opinion that they 
should be limited to purely formal ideas, without regard to the possi- 
ble origin of the structures, and that they should not be confined to 
megascopic textures, but should be used for the same kinds of struc- 
ture whether megascopic or microscopic. 
