DEFORMATION OF ROCKS 319 
tion of fan-shaped synclines. A difference in the strength of 
layers and a tendency for layers to continue to bend at certain 
places when bending has begun, rather than at other places, may 
tend to produce fan-shaped synclines. For instance, if a very 
strong layer is between two weaker layers, and this stronger layer 
becomes bent more decidedly at the outer, upper parts of the 
syncline, it may continue to bend at these places, and by its 
strength deform the softer material above and below it, so as to 
force the whole into a fan form. That minor fan-shaped syn- 
clines are thus produced is highly probable, but it may be doubted 
whether fan-shaped synclines of the first order would be thus 
formed, although they may be produced by differential thrust if 
the theory of a “level of no strain”’ be true. 
COMPOSITE FOLDS. 
The greatest flexures of the earth’s crust are termed by Dana 
geanticlines and geosynclines. Generalizing from his illustrations, it 
appears that these may be defined as flexures which are predomi- 
nantly due to the force of gravity in its tendency to produce isos- 
tatic adjustment. The deforming force is therefore mainly vertical. 
When rocks are subjected to strong lateral forces they are also 
deformed, and mountain ranges are produced. All folds, of 
whatever magnitude, thus-made by the work of great lateral 
thrust and gravity combined, when not simple, are called, follow- 
ing Dana, anticlinorra and synclinoria. An anticlinorium or syn- 
clinorium of the first order of magnitude is one which comprises 
an entire mountain range. Illustrating this usage of the terms, 
the great geological province or basin of deposition of which the 
Jura, the great valley of Switzerland, and the Alps occupy a part, 
was a geosyncline. When subjected to orogenic forces the 
mountain ranges now seen were produced. The Alps and Jura, 
taken as wholes are anticlinoria of the first order, and the great 
valley between is a synclinorium of the first order. 
The various kinds of simple folds may be united to produce 
a great variety of composite structures. A composite fold may 
be an anticlinorium or a synclinorium. An anticlinorium or 
