Dr, Hunt on the Chemistry of the Earth. 205 



quently most porous, retain at last little more than silica, alum- 

 ina and potash; while the more or less impalpable and imperme- 

 able sediments, which include large proportions of clay and marl, 

 retain their soda, lime, magnesia and oxyd of iron, and yield, 

 when metamorphosed, triclinic feldspars, pyroxene, hornblende, 

 and other minerals of basic rocks. The alteration of the more 

 silicious and thoroughly lixiviated rocks, on the contrary, will 

 yield chiefly orthoclase, mica and quartz. In this way is 

 explained the origin of the two great classes of rocks, the gran- 

 itic or trachytic, and the pyroxenic types. These two types ap- 

 pear alike among the metamorphic strata, and the intrusive 

 masses, which last we have distinguished by the title of exotic 

 rocks, regarding them as displaced metamorphosed sediments,while 

 the metamorphic strata in situ are indigenous rocks. It follows 

 as a consequence of the principles above defined, that the com- 

 position of aluminous sediments must vary in the different geo- 

 logic epochs. In the Laurentian, which is the oldest known system, 

 rocks, having abase of triclinic feldspars, which contain much 

 soda, abound, while argillaceous rocks, like argillites, are un- 

 known. These however become abundant in more recent for- 

 mations, where natriferous anorthites, like those of the Lauren- 

 tian system, are comparatively rare. (See farther Quar. Jour., 

 Geological Society, 1859, p. 488, and Am. Jour, of Science.) 



ARTICLE XXI. — Description of a new Enaliosaurianfrom the 

 Coal Measures of Nova Scotia. By 0. C. Marsh, B. A, 

 (Abridged from SUliman^s Journal.') 

 The Reptilian remains from the Coal-measures, hitherto de- 

 scribed, are few in number, and have nearly all been regarded as 

 Batrachian, or Amphibian. Previous to the year 1844, the 

 existence of even this low form of reptilian life during the Car- 

 boniferous period was unsuspected by most geologists, and its 

 first appearance upon the earth confidently referred to the Per- 

 mian epoch. In that year Herman von Meyer announced the dis- 

 covery in the Rhenish Bavarian Coal-measures of a reptile allied 

 to the Salamanders, which he described under the name Apateon 

 pedestris;^ and about the same time Dr. King published an 

 account of the footprints of a large Batrachian, which he had 



• Leonhard and Bronn, Neues Jahrbuchfiir Mineralogie, etc., 1844, page 

 33$. 



