602 STUDIES FOR STUDENTS 



alternation of layers of different characters. Such a major and 

 minor alternation of different materials may simulate the 

 appearance of bedding to a remarkable degree. If such struc- 

 tures are taken for bedding, mistakes in structural work will 

 follow. 



Where fissility is developed in an igneous rock, secondary 

 impregnations may occur between the laminae, just as above 

 described. Thus there would be formed a rock with alternating 

 layers of different mineral character, no part of which is sedi- 

 mentary, and yet which closely simulates a sedimentary struc- 

 ture. If either the sedimentary or the igneous rock which has 

 become fissile be intruded by igneous materials, these might 

 follow the cracks in a minute way, and thus again produce a 

 structure which is very similar to bedding. 



In the above cases both the process of water impregnations 

 and that of igneous injections tend to cement the rock. If the 

 process be complete the crevices of the rock may be entirely 

 healed. The once fissile rock will then have lost its fissilitv. It 

 may, however, have the property of cleavage parallel to the 

 banding. Such a cleavable rock may give no evidence that it 

 v/as once fissile. From the foregoing I conclude that banded 

 rocks may ozve their structure to fissility a?id secondary impregnations 

 or injectio7is, or both, and the bayids may or m.ay not accord with an 

 origi?ial structure. 



After a first secondary structure has developed, later move- 

 ments may produce a new cleavage or fissility, which cuts this 

 earlier structure at right or oblique angles, or the new force may . 

 be so intense as to produce a structure which wholly destroys 

 the earlier structure. Usually, in order that a new structure may 

 be produced, it is necessary that the new force shall vary con- 

 siderably in direction from the first, so that it cannot be decom- 

 posed into two components, one parallel to the old structure and 

 one at right angles to it. To this fact is doubtless due the 

 comparative frequency of cleavage and fissility in several direc- 

 tions in the same rock mass ; but while the secondary structure 

 is ordinarily in a single direction, an exposure or even a hand 



