DKFINITIOXS OF TERMS T'SKD. 19 



A region in which eruptive rocks uboimd is a region in which crystal- 

 line schists would naturally be expected to occur; f<jr hen; the conditions of 

 inetamorphisni are best developed, — conditions that ali'ect and metamor- 

 phose all the associated rocks, both eruptive and sedimentary, accordin"- to 

 their composition and physical structure. Eruptive rocks, whether in dikes 

 or lava flows, ashes or detritus of any kind, frequently possess the charac- 

 ters of crystalline schists; must they therefore be regarded as being of 

 sedimentary origin ? The writer has seen dikes of crystalline schists cut- 

 ting directly across schistose conglomerates and other sedimentary rocks. 

 Was he to conclude that these dikes of schist wxre sedimentary, and had 

 been intruded in the form of schists ; or rather that they were of eruptive 

 origin, — the original rock having been later metamorphosed into a scliis- 

 tose rock? He has also seen mica schists inclosed in distinctly eruptive 

 granites. If the law of association is worth anything, then should it not be 

 claimed that these schists were eruptive ? Ought not the evidence and the 

 facts of the origin, and not the association, to be taken as proof of what that 

 origin was ? 



At this time, when the tendency is so strong to consider almost every- 

 thing the result of stratification, it seems necessary here to call attention to 

 some characters that for the most part are common to all rocks, whatever 

 may be their origin. While these characters are taken as proof positive of 

 sedimentation, in reality they have no bearing upon the question unless 

 they are exclusively confined to one class. 



Lamination in a rock is one of these characters ; which may be defined as 

 a structure, either original or superinduced in rocks of various kinds, causing 

 them to tend to split into more or less parallel layers. 



This structure is very common in many eruptive rocks, especiidly those 

 of a fine-grained or glassy character, and which have become metamor- 

 phosed. In many rocks an appearance of lamination is brought about by 

 the deposition of coloring matters in bands, and this pseudo-lamination even 

 has been oftentimes taken for stratification. 



Joint planes may be defined as fissures traversing rocks in a regular or 

 irregular manner, independently of any other structural planes, induced 

 during the time of the consolidation of the rock or later, and dividing it 

 into masses of greater or less size.*' 



These planes are frequently mistaken for bedding planes, both in 



* Faults and fissuro veins are but luodified joints. 



