﻿Vol. 66.~] CLASSIFICATION OF IGNEOUS EOCKS. 483 



phenocrysts of sanidiue and a few flakes of biotitc, in a trachytic 

 fluidal noncrystalline ground-mass. The quartz occurs only in 

 interstitial particles. 



The intrusive bodies of this trachyte occur in various sedimentary 

 formations, the youngest being of late Cretaceous age, and side by 

 side with quartz-monzonite porphyry bodies, t} T pical of rocks wide- 

 spread in the Rocky Mountain and Plateau provinces. The 

 monchiquite dykes, some of which cut the quartz-trachyte, are all 

 small, rarely exceeding a few feet in thickness. 



In immediate association with the two rocks already mentioned 

 are other lamprophyric types, some of which are camptonitic ; 

 others are nearer to kersantite and minette. 



A few miles south of the quartz-trachyte area, in the La Plata 

 Mountains, are camptonitic dykes associated with a large number 

 of intrusive dioritic, monzonitic, and syenitic rocks in laccolithic or 

 stock intrusions. The La Plata Mountains are in fact one of the 

 laccolithic groups, like the Henry, Abajo, Carriso, El Late, and 

 Eico Mountains, where the same types of alkalicalcic magmas occur 

 in closely comparable manner. That these dyke rocks are chemi- 

 cally camptonitic seems evident from the following table of analyses 

 and norms. Mineralogically they contain abundant brown horn- 

 blende, partly in large phenocrysts, with more in the ground-mass. 

 The principal felspar is a plagioclase. 



Analyses and Norms op Camptonites. 

 Analyse*. ■ Norms. 



I. II. III. IV. I. II. III. IV. 



SiO* ... 



47-25 



43-98 



44-22 



42-73 



Ortlioclase . 



150 



9-5 



100 



12-8 



AUO., . 



1514 



1330 



1273 



1450 



Albite 



17-8 



15-7 



17-8 



21-0 



Fe 2 3 . 



505 



3-67 



5-68 



403 



Anorthite . = . 



22-8 



21-7 



20-3 



19-2 



Feb ... 



4-95 



692 



5-18 



7-28 



Nepheline... 



1-4 



1-4 





2-8 



MgO... 



6-87 



703 



6-98 



546 



Diopside ... 



21-4 



25-5 



240 



16-9 



OaO ... 



9-98 



1066 



11-57 



8-46 



Hypersthene 





... 



4-7 



... 



NaoO... 



2-39 



215 



212 



311 



Olivine 



T2 



9-3 



31 



6-8 



K 2 ... 



260 



1-64 



1-71 



228 



Magnetite... 



74 



5-3 



8-4 



5-8 



HoO+ . 



2-12 



1-521 

 042/ 



2-74 



3-08 



Ilmenite ... 



2-3 



2-3 



4-8 



8-3 



H.,0- . 



040 



0-36 



Apatite 







25 



2-5 



CO, ... 



1-87 



646 



3-66 



3-76 













Ti0 2 ... 



1-22 



1*18 



2-50 



4-30 













P2O, ... 



0-20 



0-32 



105 



0-93 













MnO... 



017 



022 



045 



0-19 













Complete analyses show small amounts of other constituents. 



I. Camptonite, Snowstorm Peak, La Plata Mts. (Colo.). Analyst, 

 W. F. Hillebrand. Bull. U.S. Geol. Surv. No. 168(1900) p. 162, and 

 La Plata Folio. 

 II. Camptonite, Indian Trail Ridge, La Plata Mts. (Colo.). Analyst, 

 W. F. Hillebrand. Bull. U.S. Geol. Surv. No. 168 (1900) pp. 162-63, 

 and La Plata Folio. G 



III. Camptonite, Kjose-Aklungen, Norway. Analyst, V. Schmelck. W. C. 



Brogger, ' Die Eruptivgesteine des Kristianiagebietes, III : Das 

 Gauggefolge des Laurdalits ' 1898, p. 51. 



IV. Camptonite, Mount Gunstock (New Hampshire). Analyst, H. S. 



Washington. ' Chemical Analyses of Igneous Rocks ' Prof. Paper 

 No. 14, U.S. Geol. Surv. 1903, pp. 318-19. 



