DEFORMATION OF ROCKS 343 
thrust was continuous or discontinuous. However, if the inter- 
vals between the successive movements were sufficiently long 
and the conditions were such that crevices could be formed and 
these were filled with secondary minerals, or if cementation or 
metasomatic changes produced new minerals, or if within the 
strata igneous rocks were intruded, the later dynamic effects upon 
such new material may enable us to determine the fact of differ- 
ent movements. 
In the Hiwassee section of the Ocoee series, in the southern 
Appalachians, in the more closely folded part of the section, quartz 
veins have formed ina first set of crevices. The rocks have 
been subsequently folded so as to closely plicate these veins, and 
after this folding a second set of unfolded quartz veins has formed. 
In other parts of the section only unfolded quartz veins are seen. 
This shows that certain parts of the area were affected by two 
periods of folding separated by a long interval. 
COMPETE EX HOD: 
ORIGIN OF COMPLEX FOLDS. 
Thus far folds have been considered in two dimensions only 
and have been treated as though they were continuous and had 
continuous axial lines, each one being a great circle of the earth. 
Such is not the case in nature. Ina given fold, in passing from 
place to place, the direction and inclination of the crest-line are 
different, and it is rarely, ifever, a part of a great circle. Its 
deviation from the horizontal at any point gives the inclination of 
the fold at that point. This inclination, measured in degrees, is 
known as the pitch. Also whena fold is followed longitudinally, 
or in the third dimension, it changes continuously in size and 
character. A fold of the greatest magnitude may be followed 
along the third dimension until it dies out. The most closely 
compressed and intricately composite fold may be followed 
in the third dimension until it becomes a gentle composite fold 
or even disappears. When a primary fold is traced in the third 
dimension it may be found to grade into asecondary fold. At 
the same time a secondary fold on the flank of a primary fold 
