GEOLOGY. 253 



north of the great peaks, and stand on the mountains bordering the 

 Tibetan plain, the snow line has receded to 19,000 or 20,000- feet. 

 This phenomenon was shown to depend chiefly on the fact that the 

 quantity of snoAV that foils to the north of the great Himalayan peaks 

 is far less than that which falls on their southern slopes. 



OX THE AZOIC SYSTEM. 



THE following is an abstract of a paper presented to the American 

 Association, Cincinnati, by J. W. Foster and J. D. Whitney, U. S. 

 Geologists. 



The term Azoic was first applied by Murchison and de Verneuil, to 

 designate a class of crystalline rocks which occur around the Gulf of 

 Finland, whose geological position is below the Silurian system. In 

 it, they include not only gneiss and mica slate, but the igneous rocks, 

 such as granite and syenite, by which they are invaded. We adopt 

 the term, but limit its signification, by applying it to a class of rocks 

 supposed to be detrital in their origin, and to have 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 sand- 

 stone. 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 immense deposits of 

 specular and magnetic oxide of iron. Most of these rocks appear to 

 be of detrital origin, but to have been greatly transformed by long-con- 

 tinued exposure to heat. They are sub-crystalline, or compact, in their 

 texture, and rarely present unequivocal signs of stratification. They 

 have been subject to the most violent dislocations. In one place the 

 beds are vertical ; in another reversed ; and in another, present a series 

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

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

 amorphism 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 

 protruded through the preexisting strata, in the form of dykes or 

 elvans ; in others, to have flowed in broad, lava streams, over the an- 

 cient surface ; and, in others, to have risen up through some wide-ex- 

 panding fissure, forming axes of elevation. 



Since the theory of metamorphism has been generally recognized, 

 many of the rocks which were formerly regarded as igneous, are now 

 referred to aqueous agency, and the transformation which they may 

 have undergone traced to the presence of erupted rocks. It is reason- 

 able to suppose that there was a time in the history of our planet when 

 its crust was subject to constantly recurring volcanic paroxysms, when 

 mephitic vapors were escaping through extensive fissures communi- 

 cating with the interior, and when the waters were in a heated con- 

 dition, and differed perhaps chemically from those of the existing 



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