594 



STUDIES FOR STUDENTS 



the formation of cross cleavage and cross fissility. It follows 

 that in heterogeneous rock strata, parallel structures may pre- 

 vail on the limbs of the fold, and cross structures on the crests 

 and in the troughs. At intervening places may be found all 

 the complex effects of the interaction of 

 the two (Fig. 7). In many cases where 

 there is almost perfect accordance of pri- 

 mary and secondary structures on the limbs, 

 and the rocks are so crystalline that the 

 two cannot readily be discriminated, at the 

 crests and troughs both structures may 

 readily be seen intersecting each other. 



Formations are but divisions of rock 

 masses greater than beds which are roughly 

 homogeneous. In each formation, consid- 

 ered as a whole, cross secondary structures 

 will usually be produced, while at the con- 

 tacts between the formations, where major 

 readjustment is sure to occur, nearly par- 

 7 (reprin e rom ^^^qI structures may be found. In the dis- 



p. 470). — Parallel fissility _ ■' 



on the limbs of the folds cussion of each separately it has been seen 



and cross fissility on the that in the cases of extreme folding the 



anticlines, and gradations j-elations between the different secondary 



between the two. After , , , ,. 



pjgjj^^ structures and beddmg are nearly the same,. 



The deformation is and therefore that cleavage and fissility 



mamly by folding, but on developed under each of the laws will merge 



the anticlines, where the , , , 11,11 • , 1 



, . , . ,, ,. , together, and both be approximately par- 



material is partly relieved o ' rr j sr 



from stress, the deforma- allcl to the reduplicated beds. They are 

 tion is partly by the mul- all brought into nearly parallel positions, 



tiple minor slips of fissility. • , 1 i 1 • .1 r ij- 



i" t" J just as are pebbles m the toldmg process. 



In order that this should be done, it is plain that there must 

 be such extreme rearrangement of the rock material that it 

 could not inaptly be compared with kneading. 



In rock masses in which the alternating layers of different 

 strength are not beds, the principles of the development of 

 cleavage and fissility are the same as in the heterogeneous 



