8 



BULLETIN OF THE 



by Messrs. Foster and Whitney, entitled, "On the Azoic System, 

 as developed in the Lake Superior Land District," in which the dis- 

 tribution of the azoic rocks in North America was briefly pointed 

 out. The term Azoic was adopted liy them from Murchinon and De 

 Verneuil,* but limited in its signification by Foster and Whitney 

 "to a class of rocks supposed to be dctrital in their origin, and to 

 Lave been formed before the dawn of animal or vegetable life. It 

 comprises the most ancient of the strata which form the crust of the 

 earth, and occupies a distinct position in the geological column; being 

 below the Potsdam sandstone. In this district, the rocks consist, for 

 the most part, of gneiss, hornblende, chlorite, talcose, and argillaceous 

 slates ; interstratified with beds of quartz, saccharoidal marble, and im- 

 mense deposits of specular and magnetic oxide of iron. Most of these 

 rocks appear to be of dctrital origin, but to have been greatly transformed 

 by long-continued exposure to heat. They arc sub-crystalline, or com- 

 pact, in their texture, and rarely present unequivocal signs of stratifica- 

 tion. They have been subject to the most violent dislocations. In one 

 place, the beds arc vertical ; in another, reversed; and in another, present 

 a scries of folded axes. Intermingled with them is a class of rocks 

 whose igneous origin can hardly be doubted, and to whose presence the 

 metamorphism so characteristic of this series is, in a measure, to be 

 ascribed. They consist of various proportions of hornblende and feld- 

 spar, forming traps and basalts; or, where magnesia abounds, pass into 

 serpentine rocks. They appear, in some instances, to have been pro- 

 truded through tlie pre-existing strata, in the form of dikes or elvans; 

 in others, to have flowed in broad lava streams over the ancient surface; 

 and in others, to have risen up through some wide-expanding fissure, 

 forming axes of elevation." Gaseous sublimations, intense pressure, and 

 eicctro-chernical agencies were thought to have assisted in the metamor- 

 phism, as well as the plutonic masses. *' Since the theory of metamor- 

 phism has been generally adopted, many of tlie rocks whicli were for- 

 merly regarded as igneous are now referred to aqueous agency, and 

 their transformations traced to the presence of erupted rocks. They 

 here cited numerous examples of metamorphism, showing that argdla- 

 ccous schist is transformed into gneiss; sandstone into compact vitreous 

 quartz; and limestone into saccharoidal marble, when brought in con- 

 tact with eruptive masses. Tliey therefore inferred, that those obscurely 

 bedded rocks, — such as gneiss, and the crystalline schists, —were of 

 sedimentary origin ; that no rock was to be regarded as igneous, unless it 



* Russia and the Ural Mouniains, I. 10. 



