586 T. C. CHAMBERLIN 
faunal distribution give evidence of minor oscillations, of advances 
and retreats of the sea, of shiftings of outlines, and of changes of 
barriers and connections, the effect of which is to introduce special 
features in the sedimentation and local or regional variations in 
the faunas. These features were subordinate to the grander 
deployments both in area and in time, and in this subordination 
they suggest that they may have been due to minor and perhaps 
separate agencies that acted more continuously and in smaller units 
—more easily and variously shifted—than the profound deformative 
agencies that were liable to interrupt the whole process. Gentler 
agencies whose general activities were harmonious with the main 
sedimentary process seem better fitted for this function than a 
supposed feeble action of a titanic agency whose normal action 
would put an end to the whole process. 
There is an aspect of continental creep, if we may follow the 
analogy of glaciers, that seems to fit it for this function. The sug- 
gestion is at least worth entertaining and testing as a working 
hypothesis. The creeping body of a glacier does not always decline 
steadily in the direction of its motion but rather assumes a more 
or less undulatory mode of progress. While the advancing surface, 
taken as a whole, slopes forward, it may, in a subordinate measure, 
slope backward, 1.e., rise in the direction of its advance. On 
broad glaciers of nearly flat surfaces, there are sometimes swells 
and sags; the surface water sometimes gathers into lakelets, and 
occasionally streamlets of notable size flow in a direction opposite 
to the flow of the glacier beneath.t These anomalies are assignable 
to irregularities in the rock floor over which the ice mass is thrust 
by its internal stresses. The creep of a continental embossment, 
if it follows the glacial analogy, may be assigned a similar undula- 
tory mode of progress. If continental creep is facilitated by a 
shear zone that had been formed previously by the powerful stresses 
that actuated the lateral crust movements of earlier times, the shear 
planes of the zone are not probably perfectly plane; they are much 
more probably undulatory, for the surface effects of the diastro- 
«See T. C. Chamberlin, “Glacial Studies in Greenland,” Jour. Geol., II (1894), 784, 
Fig. 12, view of a portion of Blase Dale Glacier showing the undulation of its surface 
involving a backward inclination. For a backward flowing stream see Vol. V (1807), 
231-32. 
