1 140 PRINCIPLES OF STRATIGRAPHY 



tana, and its continuation into Canada, with a thickness of over 

 12,000 feet, and similar formations in Utah (Uinta quartzite) 

 and in the Grand Canyon district (Unkar and Chuar series*) 

 have suffered no appreciable metamorphism, though from their 

 relationship to the overlying Cambric formations they are believed 

 to be of pre-Cambric age. The Torridon sandstone of Scotland is 

 another example of a formation underlying the Olenellus-bearing 

 sandstones (Lower Cambric), the two being separated by a slight 

 unconformity, while the American formations are generally sep- 

 arated from the Cambric only by a disconformity, or at least by an 

 unconformity in which the deformation of the lower beds has 

 been so slight as to appear non-existent. If these formations are 

 really pre-Cambric, and not basal Cambric, they may well represent 

 an earlier system, which, however, still belongs in the Palaeozoic; 

 or they may represent a pre-Palseozoic, but still post-Algonkic sys- 

 tem, one, the formation of which succeeded the apparently world- 

 wide metamorphism which has affected the Algonkic and earlier 

 formations. 



Finally, it must be noted that extensive metamorphism has af- 

 fected rocks of PalcTOzoic and even of much later age. The schists 

 and marbles of the New York City area are believed by many to 

 be the altered Cambro-Ordovicics, which, north of the Highlands, 

 appear unmetamorphosed. Berkey, however, holds that they be- 

 long to the pre-Cambric (6), a view strongly advocated by Crosby. 

 A similar difference of opinion exists with reference to the meta- 

 morphic rocks of New Jersey, especially those in the region about 

 Franklin Furnace. 



On the whole, it will be seen that correlation by metamorphism, 

 while serviceable and often very reliable within certain limits, is, 

 nevertheless, a method likely to mislead. We need but recall that 

 the early stratigraphers classed all metamorphic rocks as Pri- 

 mary, and that this included the metamorphosed Palaeozoic forma- 

 tions of western Europe, as w^ell as the metamorphosed Mesozoic 

 and later formations of the Alps. Or we may compare the older 

 and newer maps of New England and of the Appalachians, where 

 we shall find that many of the formations formerly classed as pre- 

 Cambric are now placed into the Palaeozoic. The international 

 map of Europe also shows many areas of pre-Cambric rock, where 

 more recent study has led the observers to place the metamorphic 

 formations high in the geological column. 



* In the Chuar group fossils of Palaeozoic character have been found, 

 which suggests that these formations form a pre-Cambric Palaeozoic system if 

 they are not actually a part of the Lower Cambric. 



