Revieir of Recent Geological LitevafAire. 61 
The mod(>ls and oxporimciits were desig'iied in accordanct' willi thf 
accepted conception of the eartli's crust as "a superficial sliell 5 to 7 
miles thick, which rests upon and "grades in substance and physical 
cpnditioTi into a subjacent shell. The under is only differentiated from 
the upper by its relative position in conseciuonce of which it supports a 
crushing load and forms a latently plastic foundation." I\lore tersely 
stated, the geologic condition to be imitated was that"tl»e strata which 
have suffered folding and faulting floated upon and graded downward 
into a lateirtly plastic mass. " It was further re(]uired that the thi<;k- 
ness and extent of the strata should be so related that as a whole they 
should be flexible rather than rigid. The principles which had been 
stated by-Heim. Gilbert and others, that deformation by fracture, with 
.shearing and overthrnst, occurs under moderate load, and that deform- 
ation by flexure, with open or closed and finally overturned folds, takes 
place under great load, are well proved \>y these experiments. In the 
very extensive and admirable series of illustrative plates we are shown 
the gradual development of every stage and phase in the formation of 
mountain folds and faults. 
Mr. Willis concludes that the vast lateral pressure producing the Ap- 
l)alachian and other mountain belts can not have been due wholly to 
the contraction and shrinking of the earth's interior. He would there- 
fore add to that partial explanation the theories of Dutton and Reade. 
The resulting composite theory is advocated as follows: "To every hy- 
pothesis brought forward to account for the folding of the stratified 
rocks there is one objection made by its opponents: The cause is not 
quantitatively ecpial to the task required of it. For argument's sake, 
admitting for each and every one that the criticism is sound, I do not 
understand that it disposes of any which are based on good inferences 
from observed facts. The process of deformation was exceedingly cttm- 
plex and thus afforded opportunity for the action of more than one 
cau.se. As the work performed was stupendous, it recpiired the com- 
bined [)ower of all availal^le forces." 
Referring to the yet more difficult (]uestion of the causes of the par- 
tially concomitant but (according to the opinion of Mr. Willis) unre- 
lated epeirogenic uplifts and growth of this. continent, he writes: "The 
Paleozoic continent and sea of North America had their origin in un- 
known causes of pre-Cambrian time. After Paleozoic de[)osition and 
<leformation the rise of the whole continent lifted alike the Blue Ridge 
belt of crystallines, the folded zone of the Aiii)alachian province, and 
the umlisturbed strata of the Mississippi basin. The u])lift Ijore no re- 
lation in area or time to the fact of compression, and it has gone on 
through geologic ])eriods after folding ceased, as is shown by the an- 
cient base levels and revived drainage of the whole region east of the 
Mississippi valley." To the reviewer it seems more likely that an inti- 
mate genetic relationship existed between the Api)alachian revolution 
of mountain folding and the accompanying rise of the interior of North 
America fnjm the previously very long enduring Pah'oztjic .sea; and the 
