20 THE ORIGm AND ALTERATION OF ROCKS. 



sedimentary and eruptive masses, isarticularly in regions of crystalline rocks. 

 Eruptive rocks subjected to pressure in dilces, or when tliey are metamor- 

 phosed, tend to take upon themselves a more or less parallel jointing, wliich 

 superficially resembles stratification, and "which to the writer's personal 

 knowledge has been taken for bedding planes by some of the most promi- 

 nent geologists in the United States. 



Cleavage is another structure, which by many is supposed to be confined 

 to sedimentary rocks ; but this is evidently not the case. This may be 

 defined as a tendency in rocks to split more or less indefinitely into thin 

 plates, independently of any original structure in the rock masses. In the 

 same series, the finer the grain the more perfect is the cleavage ; therefore 

 it is best developed in argillites, fine grained eruptive rocks, and volcanic 

 ashes. The presence of cleavage characters in the last two series of rocks 

 ought to make us careful in deciding upon the origin of any rocks from 

 considerations connected with their cleavage alone : further evidence should 

 be obtained, of a more decisive character. 



Foliation is another rock structure belonging to rocks of diverse origin, 

 and due either to the existence of bands or layers of different minerals, or 

 to a more or less parallel arrangement of foliated minerals, like talc, mica, 

 chlorite, etc. This structure may even be taken by rocks which are not 

 composed of foliated minerals, as, for instance, limestone. Foliated speci- 

 mens of limestone destitute of mica have been brought to the writer from 

 Western Massachusetts, as being mica schist, and considered as supporting 

 the view that in that region the limestone passes directly into mica schist. 



It is very difficult to give any definition of foliation that will cover all 

 cases of this structure, but it may in general be said to signify a structure 

 induced (not congenital) in rock masses by the arrangement of certain crys- 

 tallized minerals, or of a single mineral, in more or less parallel lines, along 

 which the crystals lie, flatways or lengthways. It has been found that in 

 sedimentary I'ocks the foliation may or may not correspond with the stratifi- 

 cation planes, and until in each case it is proved to do so it cannot be taken 

 as marking the original planes of deposition. The causes that produce 

 cleavage in some rocks seem to induce foliation in others. An example 

 of tliis is to be observed at the point of land south of Boston Harbor, known 

 as Squantnm. At this locality tlie stratification and cleavage of the argil- 

 lite and sandstone are seen to differ considerably from one another, while 

 tlie conglomerate lying between has its pebbles somewhat rearranged. 



