SOME UNSOLVED PROBLEMS IN GEOLOGY. 831 



the action of heat and heated water. Sandstones have thus passed 

 into quartzites, clays into slates and schists, limestones into marbles. 

 So far metamorphism is not a doubtful question ; but, when theories 

 of metamorphism go so far as to suppose an actual change of one ele- 

 ment for another, they go beyond the bounds of chemical credibility : 

 yet such theories of metamorphism are often boldly advanced and 

 made the basis of important conclusions. Dr. Hunt has happily given 

 the name " metasomatosis " to this imaginary and impossible kind of 

 metamorphism, which may be regarded as an extreme kind of evolu- 

 tion, akin to some of those forms of that theory employed with refer- 

 ence to life, but more easily detected and exposed. I would have it 

 to be understood that, in speaking of the metamorjohism 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 de 

 composed by atmospheric erosion can never, by any process of meta- 

 morphism, 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. Still, nothing in geology 

 absolutely perishes or is altogether discontinued ; and it is probable 

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

 Laurentian gneiss may still operate in limited localities. Then, how- 

 ever, they were general, not exceptional. It is further to be observed 

 that the term " gneiss " is sometimes of wide and even loose applica- 

 tion. Besides 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 labrador- 

 ite or anorthite instead of orthoclase, are sometimes, though not ac- 

 curately, included in the term. 



The Grenville series, or middle Laurentian, is succeeded by what 

 Logan in Canada called the upper Laurentian, and which other geolo- 

 gists have called the Norite or Norian series. Here we still have our 

 old friends the gneisses, but somewhat peculiar in type ; and associ- 

 ated with them are great beds rich in lime-feldspar the so-called lab- 

 radorite and anorthite rocks. The precise origin of these is uncertain, 

 but this much seems clear, namely, that they originated in circum- 

 stances in which the great limestones deposited in the lower or middle 

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

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

 Norian rocks to be much younger than the Laurentian, and that, as 

 Logan supposed, considerable earth-movements had occurred between 

 the two, implying lapse of time. 



Next we have the Huronian of Logan a series much less crystal- 

 line and more fragmentary, and affording more evidence of land ele- 

 vation and atmospheric and aqueous erosion, than any of the others. 

 It has great conglomerates, some of them made up of rounded pebbles 



