M4 The American Geologist. October, 1895 
attack of weather and water is still slower; but the migration of head- 
water divides by the unequal wasting of their slopes is the slowest of 
all. The occurrence of river arrangements that are indisputably due to 
this excessively slow process of migration is therefore strikingly con- 
firmatory of the doctrine of uniformitarianism. The fundamental prin- 
ciples of uniformitarianism are the postulates of the theory by which 
the spontaneous adjustment of river courses to rock structure is ac- 
counted for. The success that has been reached in explaining this class 
of natural [)henomena confirms the correctness of the postulates on 
which the explanation is based. 
Prof. Emerson, in discussion, cited instances of the robbing of one 
stream's headwaters by another in the relations of the Housatonic and 
Connecticut divides in western Massachusetts; and President Shaler 
spoke of the influence of continental tilting in bringing about such 
changes of drainage. 
Analj/sis of Folds. C. R. Van Hise, Madison, Wis. As regards 
movement, three zones in the constitution of the earth were cited, 
namely, an outer zone of fracture, an inner of fi-acture and flowage, and 
an interior one of flowage alone. The thickness of the upper zone, which 
is characterized ))y faults, will vary with the rocks. Quartzites and 
limestones, being relatively unyielding, would give the zone great thick- 
ness; but the more yielding shales and schists flow at a small depth. 
Folds are ordinarily considered as simjjle flexures in two dimensions, 
but in nature folds are compovind flexures in three dimensions. The 
analysis of simple folds given Vjy Margerie and Heim vv'as summarized. 
For the sake of simplicity, folds were first ti-eated in two dimensions. A 
composite fold is produced by the combination of various simple folds. 
Composite folds include both norma! composite folds and abnormal com- 
posite folds. The genesis of each was discussed and each was classified 
into upright, inclined, and overturned anticlinoria and synclinoria. 
When composite folds are cross folded, these are called complex folds. 
The character and origin of complex folds were discussed. Rules were 
given for observations in regions which are folded in a comjjlex manner, 
and the use of folds in the discovery of unconformity and the secondary 
changes which accompany folding were summarized. 
Prof. Davis, referring to the three zones, asked whether the speaker 
could estimate from the character of the flowage or fracture, shown by 
an eroded fold, whether much or little original burden of rock had been 
removed; and Prof. Van Hise, in rei)ly. stated that he thought it could 
be done within reasonably wide limits, as within proViably two to five 
thousand feet. 
Conditions and Effects of the Expulsion of Oases from the Interior 
atf the Earth. N. S. Shaler, Cambridge, Mass. The aim of the paper 
was to show that the phenomena of escape of gases from the earth in 
the case of ordinary springs, in the ejections of water which occur in 
•earthquakes, and in the explosions which take place in volcanic erup- 
tions, all rest on the same general basis. The column of ascent is deter- 
nained by the formation of bubbles in substantially the same way in 
