292 The American Naturalist. [March, 
historic continuity than could be admitted on the consideration of 
their enormous volumes, their disparity of age, and their widely con- 
trasted lithological facies. The fifth system is shown to lie in wide 
structural discordance with the older ones, and to be in every respect 
impossible of identification with the IVth system, with which Irving 
confounded it under the designation ‘‘ Huronian.’’ The four oldest 
systems are geographically and structurally distributed over a number 
of oval or irregular areas, each revealing granitoid rocks in the centre, 
and gneissoid rocks and crystalline schists in successive concentric 
bands, with the subcrystalline schists filling the intervals between the 
crystalline schists of neighboring areas. The granitoid and gneissoid 
centres appear to have been protruded, and crowded mutually upon 
each other, until the intervening subcrystalline strata have been thrown 
into closely folded synclinal attitudes, These schists, consequently, 
are vertical in position, and the crystalline schists and gneisses succeed 
them in close parallelism. Toward the centre of each area, the 
gneisses, even while maintaining their verticality, sustain anticlinal 
relations to each other. But sometimes, in approaching the centre, a 
dip towards the periphery supervenes, and this diminishes to horizon- 
tality. The centre of the area is not always occupied by granitic 
rocks. In some cases, they are strictly gneissoid. The state of the 
facts may be easily comprehended by conceiving,all the systems origin- 
ally in a horizontal position, and pasate) damm abe in the order 
indicated—then an up-swelling in places, which, with shrinkage of the 
terrestrial crust, would crowd the higher beds into interareal synclinal 
-olds as we find them. 
Thus, if we had only the geology of the Northwest to settle, the 
Archean question would be simple. Older observations in more diffi- 
cult regions have created questions of correlation which the canons of 
science require us to answer. But these and other questions of a 
speculative character are postponed to future occasions. 
ORIGIN OF THE ROCK-PRESSURE OF NATURAL GAS IN THE TRENTON 
LIMESTONE OF OHIO AND INDIANA. Edward Orton, Columbus, Ohio. 
—In this paper the author defines the term rock-pressure: shows the 
decrease in rate westward ; discusses theories of origin already offered : 
presents facts supporting the theory of hydrostatic origin: offers con- 
clusions respecting duration of gas-supply in Ohio. 
On Por-HoLes Nortu or LAKE SUPERIOR UNCONNECTED WITH 
Existinc Streams. Peter McKellar, Fort William, Ontario.—This 
paper deals with some large and remarkable “ Kettles” or “ pot-holes’’ 


