70 NEW YORK STATE MUSEUM 



the formations in which they are found if one takes their history 

 into account. They do give, however, the impression of hopeless 

 confusion and a certain vagueness of character which adds much 

 to the difficuhy of satisfactory field determination. Not only is the 

 identification of a formation obscured by this variability, but its 

 boundaries are to a large degree uncertain and the identity and inter- 

 pretation of a given occurrence are frequently difficult or impossible. 



Certain structural characteristics may be credited to syntexis or 

 absorption. One of these is a gneissoid structure where the original 

 country rock itself had pronounced structure and especially if it 

 was made up of differing streaks or bands (plate 41). 



A result much less uniform than the gneissoid habit is derived 

 from the incorporation of massive material, and this does not 

 develop gneissoid structure without accompanying magmatic move- 

 ments. The more common effect in such case is a patchy habit of 

 the invading rock not caused by simple differentiation. With mag- 

 matic movement it is believed that syntectic portions of the magma 

 may develop striking structural qualities, simulating even the definite 

 banding of an injection gneiss. 



The injection type of structure, however, is usually much sharper, 

 and, in the ideal case, has determinable differences of material in 

 the alternating bands. In the typical case, injection may not involve 

 much absorption of the invaded walls or much syntexis, and it ought 

 to be possible to distinguish rather sharply the character of the 

 injection material, connecting it directly with its true source. As a 

 matter of fact, there are all gradations between complete syntexis 

 and clear-cut simple injection, and the commoner occurrences are 

 intermediate in behavior. They have certainly accomplished a good 

 deal of modification of the invaded rock, and have evidently worked 

 over portions of the walls and to some degree invaded the weak- 

 nesses of these walls, so that everywhere the invasion matters are 

 intimately intergrown with or mixed with the original rock material. 

 It is perfectly clear in great numbers of outcrops that an injection 

 process has given the fine strikingly banded structures characterizing 

 them. The clearer cases are those in which some transverse cutting 

 of the formation occurs in addition to the banding which usually 

 follows accurately the original structure. This cross-cutting of the 

 structure is always taken as proof of invasion origin, v/hereas simple 

 banding in the absence of such supplementary evidence has at least 

 other possibilities. 



Similar structural habit may possibly be derived by differentiation 



