BY H. I. JENSEN. 145 



Cryptographic intergrowths of two felspars, apparently both 

 sanidine, are also of common occurrence in the base of the Ngun- 

 Ngun and Conowrin lavas. These are very like the granophyre 

 groups described by Iddings.* 



Chemical Investigation of the Trachytes and 

 Allied Rocks. 



Beerwah trachyte, Conowrin and Coolum comendites and 

 pantellarites from Trachyte Range (Skeleton Cave) and Mt. 

 Ngun-Ngun have been analysed, and the results, as well as the 

 norms calculated from the analyses, are stated in tabular form on 

 pp. 168-171. These rocks were all, prior to chemical investigation, 

 looked upon by me as trachytes. The amount of quartz recog- 

 nisable as such under the microscope is very small indeed, yet the 

 analyses of the comendites and pantellarites mentioned show that 

 there must be in these rocks a considerable amount of free quartz; 

 it is found to be present mainly in the microfelsitic base, partly 

 in cryptographic intergrowth with felspar. It is consequently 

 necessary to remove these orthophyric rocks from the trachytes, 

 with which I classed them in my previous paper, and to place 

 them with the comendites and pantellarites of Rosenbusch, near 

 which they fall.f 



All the above-mentioned rocks are very rich in alkalies, 

 especially soda. Consequently the microscopic evidence that the 

 sanidine is partly anorthoclase has strong chemical support. The 

 mode differs from the norm in that the diopside, hypersthene, 

 magnetite, hematite, and ilmenite molecules have all gone into 

 soda hornblendes, like riebeckite, arfvedsonite, barkevicite and 

 cossyrite. 



That both arfvedsonite and riebeckite are present is evident 

 from the analyses. The blue hornblende is practically the only 

 ferromagnesian mineral in these rocks worth considering. If 

 this mineral were only riebeckite we should expect less FeO, as in 



* U.S. Geol. Surv. Seventh Ann. Kep. 1885-86 "Obsidian Cliff." 

 t See ' Elemente der Gesteinlehre ' by Rosenbusch, pp.267 to 270. 

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