210 The American Natu 





Rocks from the Sweet Grass Hills, Montana.— Weed and 

 Pirsson 6 describe the rocks of the Sweet Grass Hills of Montana as 

 quartz-diorite-porphyrites, quartz-syenite-porphyries and minettes. The 

 first named rock presents no special peculiarities. The quartz-syenite- 

 porphyry contains orthoclase, plagioclase and augite-phenocrysts in a 

 fine groundmass of allotriomorphic feldspar and quartz. The augite 

 is in short thick prisms composed of a pale green diopside core, which 

 passes into a bright green aegerite mantle. The miuette also contains 

 aegerine, but otherwise it is typical. 



Petrographical News.— Two peculiar phonolitic rocks are de- 

 scribed by Pirsson 7 from near Fort Claggett, Montana. One is a 

 leucite-sodalite-tinguaite, with leucite pseudomorphs, and sodalite as 

 phenocrysts in a groundmass composed mainly of a felt of orthoclase 

 and aegerine. The leucite pseudomorphs are now an aggregate of 

 orthoclase and nepheline. In the centers of some of them are small 

 stout prisms of an unknown brown mineral, that is pleochroic in brown- 

 ish and yellowish tints. The second rock is a quartz-tinguaite porphyry 

 somewhat similar to Brogger's grorudite. 8 



In a few notes on the surface lava flows associated with the Unkar 

 beds of the Grand Canon series in the Canon of the Colorado, Ariz., 

 Iddings 9 briefly describes compact and amygdaloidal basalts and fresh 

 looking dolerites that are identical in all respects with modern rocks of 

 the same character. 



Laspeyres 10 estimates that the quantity of carbon-dioxide in liquid 

 and gaseous form contained in rocks is sufficient to serve as the source 

 for all that which escapes from the earth's natural fissures as gas, as 

 well as that which escapes in solution with spring water. It may be 

 set loose from the rocks through the action of heat or through the 

 action of dynamic forces. 



In a handsomely illustrated brochure Merrill 11 describes the charac- 

 f the onyx marbles and the processes by which they originate. 

 1 temperature, according to the author, are not the control- 

 determining the differences in texture between the 

 avertine. He is inclined to the belief that the banded 

 onyxes were formed by deposition from warm solutions under pressure 

 flowing into pools of quiet cold water. 



•Amer. Jour. Sci., Vol. I, p. 309. 



7 Amer. Journ. Sci., 1895, Nov. p. 394. 



8 American Naturalist, 1895, p. 567. 



9 14th Ann. Rep. U. S. Geol. Survey, p. 520. 

 iturh. Ver. preus s . Rheinl., No. 2, 1894, p. 17. 



