232 A. P. COLEMAN—CLASTIC HURONIAN ROCKS. 
deposited on a sea bottom, since entirely destroyed. The adjoining land 
must have presented an abundance of quartz-bearing rocks to furnish 
so many cubic miles of sand. 
After at least a mile and a half and probably five miles thickness of 
these sands had been laid down in the southern portion of the region, 
the secona or Keewatin series began, in which great eruptions of basic 
and acid rocks alternate with clays, grits, and conglomerates, the latter 
sometimes a mile thick. It is probable that an important break in the 
series is shown by the conglomerates. During the later part, if not the 
whole, of the time we may suppose that organisms existed, furnishing 
the large percentage of carbon found in some of the rocks. : 
Ultimately the sinking sea bottom was loaded with an eight or ten miles 
thickness of sediments and eruptive materials, as in the geosynclines 
preparing the way for later mountain ranges, and the slowly rising iso- 
geotherms softened or fused the foundation, which rose into domes, the 
inner parts solidifying as granite, the outer more viscid portions having 
their constituents dragged into rough parallelism with the adjoining solid 
rocks and forming gneiss. 
COMPARISON WITH OTHER REGIONS 
The usual theory of mountain-building, by lateral thrust due to the 
sinking in of the earth’s crust to conform to the shrinkage of the interior 
through loss of heat or of volatile constituents, seems capable of produc- 
ing only folds; and it is doubtful if thrusts in two directions at right 
angles to each other could produce anything except more complex forms 
of the same kind. The formation of irregularly placed domes demands 
some other cause. { 
One naturally compares these batholitic* mountains with the lacco- 
lites so distinctly brought before the world by Gilbert in his description 
of the Henry mountains of Colorado. ‘There also there are oval domes, 
though not over 3 or4 miles in diameter. Larger but more irregular ones 
are described by Whitman Cross from the adjoining western states. It 
is evident, however, that these cake-like masses of eruptive rock resting 
on their undisturbed floor of stratified rock differ greatly from the Rainy 
Lake mountain stumps, which are only more elevated portions of the 
general substratum of gneiss on which the sedimentary rocks now rest. 
The eruptive masses described by Dawson from the Sweet Grass hills 
of northern Montana, tilting up “the previously horizontal beds of the 
plains,” so that those immediately surrounding the igneous masses rest 
* Bathylite is the form of the word preferred by Dana and Zirkel. 
