AN ANALCITE-BEARING CAMPTONITE 



505 



has compared the slides of the Las Vegas rock with those from 

 the Champlain region, and also with those from Campton Falls, 1 

 from Whitehall and Fairhaven, Vt., 2 and several neighboring 

 localities ; and with a series from the Black Forest in Germany. 

 When all these camptonites were studied and compared with 

 the Las Vegas rock, 

 a marked difference 

 in texture was seen. 

 The Las Vegas rock 

 is coarser grained, 

 the phenocrysts more 

 abundant, and there 

 is a less marked 

 difference in size be- 

 tween phenocrysts 

 and groundmass. 

 This is what might 

 be expected since the 

 eastern rocks occur 

 in dikes, and the Las 

 Vegas one in a stock. 

 The eastern rocks are 

 all considerably 

 altered, containing 



calcite, serpentine, delessite, and secondary analcite. The Las 

 Vegas rock is remarkably fresh. This comparison gives strong 

 indirect evidence of the primary character and analcitic nature 

 of the isotropic substance ; the rock is too coarse-grained for it 

 to be a glass and too fresh for it to be a secondary product. 



The term " Camptonite " is commonly applied to the plagio- 

 elasic lamprophyres, in distinction from minette and vogesite 

 which are orthoclasic. Common usage 3 has restricted the term 

 Camptonite to the lamprophyric plagioclase rocks with horn- 



1 Described by J. W. Hawes, Amer. Jour. Set., ser. 3, Vol. XVII, p. 148. 



2 Described by J. F. Kemp, Amer. Geo/., Aug., 1889. 



3F. Bascom, Nineteenth Ann. Rept. U. S. Geol. Surv., Part III. 



Fig. 4. — Typical section of the Las Vegas Camp- 

 tonite, showing phenocrysts and groundmass. 



