WORLD-MAKING 21 



metamorphism. I would have it to be understood that, in 

 speaking of the metamorphism of the older crystalline rocks, it 

 is not to this metasomatosis that I refer, and that I hold that 

 rocks which have been produced out of the materials decom- 

 posed by atmospheric erosion can never by any process of 

 metamorphism be restored to the precise condition of the 

 Laurentian rocks. Thus, there is in the older formations a 

 genealogy of rocks, which, in the absence of fossils, may be 

 used with some confidence, but which does not apply to the 

 more modern deposits, and which gives a validity to the use of 

 mineral character in classifying older rocks which does not 

 hold for later formations. Still, nothing in geology abso- 

 lutely perishes, or is altogether discontinued ; and it is prob- 

 able that, down to the present day, the causes which produced 

 the old Laurentian gneiss may still operate in limited locali- 

 ties. Then, however, they were general, not exceptional. It is 

 further to be observed that the term gneiss is sometimes of wide 

 and even loose application. Beside the typical orthoclase and 

 hornblendic gneiss of the Laurentian, there are micaceous, 

 quartzose, garnetiferous and many other kinds of gneiss ; and 

 even gneissose rocks, which hold labradorite or anorthite in- 

 stead of orthoclase, are sometimes, though not accurately, in- 

 cluded in the term. 



The Grenville series, or Middle Laurentian, is succeeded by 

 what Logan in Canada called the Upper Laurentian, and which 

 other geologists have called the Norite or Norian series. Here 

 we still have our old friends the gneisses, but somewhat peculiar 

 in type, and associated with them are great beds and masses, rich 

 in lime-felspar, the so-called labradorite and anorthite rocks. 

 The precise origin of these is uncertain, but this much seems 

 clear, namely, that they originated in circumstances in which 

 the great limestones deposited in the Lower or Middle Lauren- 

 tian were beginning to be employed in the manufacture, prob- 

 ably by aqueo-igneous agencies, of lime-felspars. This proves 



