﻿482 DR. WHITMAN CROSS ON THE NATURAL [Aug. I9IO, 



in a form which will convince the partisans of that belief. To 

 quote from an experienced correspondent on this matter :— 



' If you show that there is no large mass to connect the dykes with, they say 

 it is underground and not exposed by erosion ; if you present evidence that 

 this is improbable, then the province is an alkalic one anyhow and such rocks 

 are to be expected. If you show that the dykes are associated with a kind of 

 rock different from what was to be expected, you are informed that you are 

 mistaken ; that you have not understood the real nature of the rock-mass, 

 that it is, for example, not a diabase or gabbro, as hitherto regarded, but really 

 an essexite. Or, if tbe parent mass is too plainly fixed to shift its base, then 

 you are mistaken in your idea of the dykes — they are not actually camptonite, 

 but a variety of spessartite with brown hornblende ! ' 



This situation arises from the indefinite, arbitrary distinction 

 between the two magmatic series above discussed. 



Certain ' dyke rocks' of Colorado. — We may pass now to 

 a specific case of notable exception to the rule, postulated by llosen- 

 busch, that bostonite, camptonite, and monchiquite magmas belong 

 exclusively to the ' Gefolgschaft ' of foyaitic-theralitic parent 

 magmas. In the Engineer Mountain quadrangle of South- Western 

 Colorado there occur several thin laccoliths and sills of a rock which 

 I call ' quartz-trachyte.' In the immediate vicinity are numerous 

 small dykes of typical monchiquite and some of the camptonitic 

 rocks, and others. While the region is rich in all manner of 

 Tertiary igneous rocks of the ' granito-dioritic series ' there are 

 none of the •' foyaitic-theralitic scries '. No analysis has been made 

 of the monchiquite, but the quartz-trachyte has the composition 

 stated in column I of the following table, while in column II is 

 tabulated the analysis of the typical bostonite of Marblehead Neck 

 (Mass.). In adjacent columns are tabulated the calculated norms 

 of the two analyses. 



Table of Analyses and Norms. 

 Analyses. Sunns. 



I. II. I. II. 



SiO 



7073 



034 



7023 

 003 

 15-00 

 1-99 

 vmdet. 

 0-24 

 0-38 

 0-33 

 4-98 

 4-99 

 0-91 

 1-28 

 006 



Quartz 



. 19-6 

 . 334 



. 41-9 



.' 3 : 8 



196 



TiO 



Ortboclase 



29-5 



Al 



14-22 



T59 



Albite 



41-9 



Fe 



Anorthite 



17 



Feb 



MnO 



MgO 



CaO 



0-59 



0-11 



none 



072 



Corundum 



Femic molecules . 



0-8 



41 



Na.,0 



K,0 



H~0- 



4-96 



5-57 



1-16 





H"0 + 



0-32 





PAX 



003 





I. Quartz-trachyte, Grayrock Peak, Engineer Mountain quadrangle 

 (Colorado). Analysis by George Steiger. 

 II. Bostonite, Marblehead Neck (Mass.). Analysis by T. M. Chatard. 



The quartz-trachyte is nearly identical chemically with the type 

 bostonite. The name chosen expresses the fact that it has a strong 

 textural resemblance to trachyte, since it contains numerous tabular 



