144 METAMOEPHIC EOCKS 



by many geologists as representing '* portions of the primeval 

 crust of the globe, traces of the surface that first congealed upon 

 the molten nucleus/' By others they are regarded as meta- 

 morphosed sedimentary deposits resulting from the breaking 

 down of still older rocks, and may not in themselves, therefore, 

 be confined to any particular geological horizon. They are in 

 large part, however, unquestionably the oldest known rocks, 

 lying beneath or being cut by all rocks of later formation or in- 

 jection. , 



The origin of the gneisses, as already suggested, is in many 

 cases somewhat obscure, the banded or foliated structure being 

 considered by some as representing the original bedding of the 

 sediments, the different bands representing layers of varying 

 composition. This structure is now however, considered to be 

 due to mechanical causes, and in no way dependent upon origi- 

 nal stratification. The name, as commonly used, is made to in- 

 clude rocks of widely different structure, which are beyond doubt 

 in part sedimentary and in part eruptive, but in all cases 

 altered from their original conditions. 



This alteration, it should be stated, has been brought about 

 not by heat and crystallization alone, but in many cases by 

 processes of squeezing, crumpling, and folding so complex as 

 almost to warrant the application of the term kneading. It is 

 even possible to conceive that some of them may be original 

 massive or foliated rocks into which eruptive materials have 

 since been injected along lines of foliation or of weakness due to 

 shearing, and the entire mass again submitted to such a knead- 

 ing as to render it practically impossible to now decide what 

 are portions of the original rock and what of the subsequently 

 injected. 



The close chemical relationship which may exist between 

 clastic, metamorphic, and eruptive rocks is shown in the selected 

 series of analyses given on the following page. 



Figures 1 and 2 on PL 12 show two rather extreme types of 

 these gneissoid rocks. Figure 1 is that of a banded gneiss from 

 Madison County, Montana. In Fig. 2 is shown a foliated rather 

 than a banded rock, and whatever may have been its origin, it 

 undoubtedly owes its foliated structure to dynamic agencies. 

 The effect of the shearing force whereby the foliation was pro- 

 duced is evident in the figure to the left and just above the 

 centre, where an elongated feldspar is seen broken transversely 



