PRE-CAMBRIAN ROCKS. 103 



Dalradiaii of Scotland. Some portions of these rocks may, however, be 

 the same with what in Canada has been called b}^ Matthew " Basal 

 Cambrian." 



It is evident that in Scotland, as in North America, the Laurentian 

 rocks have been elevated into land before the deposition of the Huronian, 

 and that the latter and the Kewenian are coarse littoral deposits clinging 

 to the Laurentian shores, protected in part from lateral crushing by their 

 hard Laurentian base, and represented at a greater distance froui the old 

 land by formations which have sometimes received different names, and 

 which are usually in a state of greater alteration and compression. 



It may be remarked here that in Canada, though the Laurentian beds 

 are much folded and contorted, they are comparatively little affected by 

 faults or overthrusts, and the succession is often extremely clear, while 

 the outcrops of individual beds can be traced over great distances. This 

 applies especially to the Upper or Gren\dlle series, holding the great 

 limestones and beds of graphite and magnetite and the serpentinous lime- 

 stone containing eozoon. 



The simple arrangement of the infra-Cambrian rocks as Kewenian, 

 Huronian and Upper and Lower Laurentian is further ^T.ndicated by 

 Walcott's section in the Colorado canyon, which shows them not only 

 superimposed but unconformable. The lowest member is a granitic 

 rock probably equivalent to the fundamental gneiss. Walcott has found 

 in the upper part of the infra-Cambrian an obscure discina-like or steno- 

 theca-like shell and a fragment resembling the cheek of a small trilobite. 

 Still lower are the stromatoporoid masses of supposed Cryptozoum. Some 

 specimens of this, recently sHced, show distinct traces of structure similar 

 to that of Hall's typical species of Cryptozoum. 



From long acquaintance with these rocks I conclude that the fourfold 

 arrangement of Lower Laurentian, Upper Laurentian, Huronian and 

 Kewenian will include them all, and that the name Algonkian, recently 

 proposed, is merely provisional and equivalent to pre-Cambrian, which 

 has been used to include rocks of uncertain classification in the base of 

 or older than the Paleozoic. 



Mountain-making. 



It is an easy transition from the old crystalline rocks to the mountain 

 masses which so largely consist of them, and our knowledge of the fold- 

 ings, crumplings and overthrusts of the older rocks certainly gives much 

 help in the comprehension of mountain-making. Yet we must not for- 

 get that all mountains are not made up of old rocks folded and pushed 

 over or under each other. Mountains of great magnitude, Hke Etna, 



