REPORT OF TEE CHIEF ASTRONOMER 407 



SESSIONAL PAPER No. 25a 



Chemically the rock of the chonolith is most nearly matched by the ' basic 

 syenite ' and ' syenite-porphyry ' from the Highwood mountains, as described 

 by Pirsson.* The average of four analyses of the latter rocks is noted in Col. 

 6 of Table XXVI. Pirsson's list of the component minerals is : iron ore, apatite, 

 biotite, olivine, augite, orthoclase often surrounded with rims of soda-orthoclase, 

 and sometimes demonstrable nephelite. In the Highwood porphyry the feldspar 

 phenocryst-s have tabular habit and do not form rhombs. Anorthoclase is not 

 described as occurring in any of the four analyzed types. The relatively high 

 barium and strontium oxides suggest that the alkali feldspars, as in the chono- 

 lith porphyry, are charged with the celsian molecule at least, but the orthoclase 

 is described as free from an appreciable admixture of the anorthite molecule. 



In spite of the differences noted, the rock of the chonolith must be regarded 

 as remarkably similar to the uncommon basic syenite of the Montana occur- 

 rence. In the Norm classification, the two enter the same subrang, namely, 

 the sodipotassic borolanose, of the domalkalic rang, essexase, in the dosalane 

 order, norgare. The norm of the chonolithic rock has been calculated as fol- 

 lows : — 



Orthoclase 33-92 



Albite 29-43 



Anorthite 11-40 



Xephelite 10-51 



Diopside 6-32 



Hypersthene - .. 5-60 



Magnetite 2-09 



Hematite 2-88 



Ilmenite 1-67 



Apatite 1-24 



Water 3-42 



, 99-48 



Mineralogically and structurally, however, the nearest relative to the 

 chonolithic rock is tbe rhomb-porphyry described by Brogger and others, and 

 the rock may be referred to henceforth under that name. In the Norm classi- 

 fication the typical Norwegian rhomb-porphyry is referred by Washington to 

 laurvikose, the dosodic subrang of the perfelic, persalane order, canadare. 

 Broggers * nephelite rhomb-porphyry ' enters the dosodic subrang, viezzenose, 

 of the peralkalic rang, miaskase, in the lendofelic, persalane order, russare. 

 The three types thus represent three different subrangs in as many different 

 rangs, orders, and classes. The norm-chemical system of classification evi- 

 dently fails to bring out those similarities among the three types which must 

 impress the field-geologist in the highest degree. The older, more purely 

 mineralogical classification, laying emphasis on the dominance of the rare 

 feldspar, anorthoclase, in the at least equally rare rhombic form, obscures the 

 obvious differences in magmatic relationships. Neither as borolanose nor as 

 rhomb-porphyry is the British Columbia rock ideally classified. That can only 

 be done when what may be called a mode-chemical system of classification is 



* L. V. Pirsson, Bull. 237, U.S. Geol. Surv., pp. 89 ff, 1905. 



