342 STUDS FORM SIRO EANAGS 
synclinorium. The abnormal character is due to the very great 
difference in strength between the Archean granite and the 
Algonkian Lower Marquette formations. At the east end of 
the Marquette district, where the Archean rocks are relatively 
weak schists, the synclinorium is normal. 
Limit of term fold.—By the above analysis and examples it is 
not meant to imply that, where sediments were deposited off a 
land area and the land and sea areas were afterwards compressed, 
producing undulations in the sedimentary rocks, and often in 
those also of the original land areas, the terms anticlinorium and 
synclinorium are properly applicable to the primary flexures. 
Following Dana, as indicated on a previous page, such flexures 
are more properly described as geanticlines and geosynclines. 
In such cases, however, there is a differential uprising or subsi- 
dence, in many cases producing a composite flexure. It is 
believed that the principles applied to primary folds of true anti- 
clinoria and synclinoria apply to the secondary folds in question. 
just as though they together constituted a part of an ordinary fold. 
When gravity controls their form they are normal. When the 
difference of strength of the rocks controls their form they are 
abnormal. 
As a matter of fact, it is often difficult to determine ina given 
mountain range whether the so-called core rocks were deeply 
buried under a great thickness of sediments, being, perhaps, in 
or near the center of the trough of deposition, or whether they 
were, and continued to be, land areas. It is thought that it 1s 
one of the advantages of the treatment given that it is not neces- 
sary to decide this question before working out the structure of 
the district. In either case the types of flexures of the second 
and higher orders formed on the primary flexures and the laws | 
controlling them are the same. 
Movements continuous or discontinuous.—Composite folds may be 
the result of forces acting continuously or discontinuously. The 
different secondary folds may develop at different times. The 
higher orders of folds may begin to form only at a late stage of 
development. Usually it is impossible to determine whether 
