314 Spurr — Scapolite Rocks from Alaska. 



In both these cases the scapolite is regarded as original. 

 There is no evidence of its derivation from feldspar, but rather 

 in every case of contemporaneous formation. Certainly the 

 mineral is not the product of weathering, for it is nearly 

 always in a process of decomposition, showing itself as unstable 

 under atmospheric conditions as are the feldspars. It is exceed- 

 ingly probable, however, that in the formation of this, as in 

 other occurrences of scapolite rock, gases have played an 

 important part. The scapolites contain chlorine, otherwise 

 they have essentially the composition of the soda-lime feld- 

 spars. The marialite scapolite corresponds to oligoclase, and 

 the meionite scapolite to anorthite, with a series between like 

 that of the feldspars, so that when scapolite is found instead of 

 feldspar we may suppose chlorine gas to have been present at 

 the time of the formation of the rock. This is, however, no 

 good reason for considering the rock as of secondary origin, 

 for in most igneous rocks the gases play some part in the 

 formation and in many a very weighty part, the rocks never- 

 theless belonging to the class of original igneous rocks. The 

 writer is inclined, therefore, to give the scapolite rocks a place 

 among the primary igneous rocks wherever the scapolite shows 

 evidence of having formed contemporaneously with the other 

 rock minerals. In cases where it evidently is an alteration 

 product from some other rock} the classification will of course 

 be different. 



In case classification is attempted, the scapolites might be 

 considered, for the purposes of classification, as equivalent to 

 the feldspars, when they occur in rocks where feldspar is 

 present in an equal or greater amount. Thus the biotite-scapo- 

 lite rock from the Yentna River might be called a biotite 

 scapolite-belugite,* since the feldspar belongs to the andesine- 

 oligoclase series, and the equally important scapolite seems to 

 be of a variety containing much lime and corresponding to 

 anorthite in the feldspar series. The scapolite-belugites might 

 then be considered a group of the belugite family. 



The porphyritic scapolite rock from the mouth of the Holik- 

 nuk on the Kuskokwim might be called a scapolite-adamellite 

 porphyry, adamellite being the name adopted by Brogger for 

 the quartz-bearing monzonites. In this case the scapolite 

 appears to belong mainly to the soda end of the soda-lime 

 series, and thus is the equivalent of oligoclase-andesine. The 

 groundmass appearing to be mainly orthoclase, the feldspathic 

 constituents (considering scapolite as such) would be on the 

 whole intermediate between the granite and diorite families, or 

 in the monzonite family. In this case the rock might be con- 



* See writer in Am. Geol., xxv, April, 1900, p. 233. 



