30 THE CANADIAN NATURALIST. [Feb. 



tinguished by its somewhat greater density (2*75). It consists 

 essentially of labradorite, with a little quartz, pyroxene, olivine, 

 and magnetite. (Bui. Soc. Geol. de France [2], viii, 425.) Hum- 

 boldt designates the trachy-dolerites of Etna and of the Peak of 

 Teneriffe as trachytes (Comptes Rendus, xliv, 1067) ; so that this 

 word, like porphyry, comes to indicate nothing more than a pecu- 

 larity of structure, which may be assumed by various feldspathic 

 rocks. The trachytic orthosites, as we have seen, pass into gran- 

 ites, from which they do not differ in chemical composition ; and 

 their differences in texture probably depend upon the fact that the 

 one was solidified under great pressure, and the other near the sur- 

 face, trachytes passing in fact into lavas. The observations of Sorby 

 on the fluid-cavities in the crystals of granites and of trachytes 

 are in point. 



Among the intrusive rocks of Canada to be described are granitoid, 

 compact, and earthy varieties of trachytic orthosites, besides tra- 

 chytic porphyries. These rocks often contain disseminated earthy 

 carbonates, sometimes in considerable amount; as Deville had 

 already shown for some of the trachytes of Hungary, and as I have 

 also observed for those of the Siebengebirge on the Rhine. Tra- 

 chytes also hold in some cases disseminated portions of a zeolite, 

 apparently natrolite ; and through this mixture pass into phono- 

 lites, of which a characteristic variety will be noticed in this paper. 

 Obsidian and pumice-stone, which are often associated with ortho- 

 clase trachytes, are related to them in composition ; and pitchstone 

 and perlite are similar rocks, differing however in containing some 

 combined water. Rocks resembling pitchstone, and sometimes 

 porphyritic from the presence of distinct crystals of feldspar, occur 

 in the south side of Michipicoten Island, Lake Superior, but have 

 not yet been examined. (Analyses by Jackson and by Whitney of 

 thepitchstonesof Isle Royale will be found in Silliman's Journal 

 [2], xi, 401; xvii, 128.) 



The presence of an anorthic feldspar, generally oligoclase, in 

 many granites and trachytes, not less than the admixture of or- 

 thoclase crystals in some of the trachytic dolerites of Etna, serves 

 to connect the orthosite with the anorthosite family. Great masses 

 of indigenous rock in the Labrador series in Canada, are made up of 

 almost pure granular labradorite, or related triclinic feldspars, and 

 might be termedjnormal anorthosites. (Silliman's Journal [2], xxxvi, 

 224; Geol. of Canada, 588.) In most cases however, these feld- 



