FOLIATION — FLITIDAL STRUCTURE. 21 



srivins: rise to a schistose or semi-foliated structure. These foliation planes 

 correspond with the cleavage planes, but not with the original bedding of 

 the rock. 



Foliation is of frequent occurrence in metamorphosed eruptive rocks, 

 givins; rise to schistose forms which are ordinarily taken for micaceous, 

 chloritic, and otlier schists, which are usually regarded as sedimentary. 

 All these are cases of similar structures and similar rocks, resulting from 

 the alteration of rocks of diverse origin, but of similar chemical com- 

 position. Foliation usually corresponds to the lines of pressure, — either 

 from the walls or from the surface downward, and is usually brought about 

 by recrystallization of the rock constituents during the process of altera- 

 tion. Since rocks of every kind are subject to metamorphism, and none 

 more so than the basic eruptive ones, it is natural to suppose that highly 

 altered eruptive as well as sedimentary rocks would display that character 

 of structure which is designated by the term foliation. 



Many rocks which can hardly be said to possess lamination or foliation, 

 show nevertheless a schistose or fissile structure, which is a property of all 

 moi'e or less altered rocks, — a superinduced structure, and not a congenital 

 one. Hence, this structure caimot be taken as being confined to a single 

 class of rocks, and therefore a diagnostic for that class, as has been done by 

 some geologists. 



Fluidal structure is a character induced in eruptive rocks through their 

 having moved or flowed when in a licpiid or pasty condition. It is best seen 

 in the glass}- and acidic eruptive rocks and furnace slags. This structure 

 belongs to, and is characteristic of eruptive rocks. It is by no means con- 

 fined to lava flows, but is also to be seen in dikes. The difficulty in regard 

 to employing it as a diagnostic character arises from the close resemblance 

 of it to other structures in altered rocks, and its obliteration by secondary 

 alteration in the rock mass. A schistose structure induced in rocks by alter- 

 ation is the one structure that under the microscope is most often mistaken 

 for fluidal structure. Fluidal structure has been taken for bands of sedi- 

 mentation in a large nundjer of instances, particularly in the older acidic 

 rocks like felsite and granite. 



Tlie bands of chemical deposition, as in the case of silica from hot springs, 

 have in some instances been confoimded with the fluidal structure of erup- 

 tive rocks, although distinct from that both in character and cause. 



In the employment of the fluidal characters in the older rocks, great 



