Hittebrand and Dana — Note* an VyrMUfrtm Utah. k 2Tl 



daring the section much less transparent The rooks, however, 

 are all of the same type and their differences so Blight that they 

 very probably all belong to the same system. 



A specimen collected by Mr. II. \l. Geiger from ;i dike in 

 limestone of the valley near Brook's Gap, 5 miles northwest of 

 Harrisonburg, Ya.. is closely related in composition to those 

 collected by Mr. Darton farther to the southwestward. It is 

 holocrystalline but not porphyritic, and characterized by the 

 Btrnctnre which Jndd has designated ophitic. The magnetite 

 and granular olivine arc much less abundant than the augite 

 and the lath-shaped plagioclase. The locality last named is 

 nearer the Triassie than those in the region of Monterey. At 

 a point 3 miles Bonth of Lbvettsville, Loudon Co., Va., in the 

 Blue Ridge district and nearer the Trias than any of the other 

 localities, Mr. Geiger discovered another dike from which he 

 collected a hand specimen that in every way resembles the 

 Triassie eruptives. This similarity is fully borne out by a 

 microscopic examination which shows the rock to be composed 

 ntially of augite and plagioclase, and to have a regular 

 ophitic structure like the majority of those within the Mesozoic 

 belt. 



]t is well known that the eruptives associated with the Trias 

 of the Atlantic slope from Massachusetts to North Carolina 

 extend beyond the limits of that formation in Pennsylvania and 

 several other States, but as yet it appears that comparatively 

 little is known of their real distribution. Their wide extent 

 and remarkable uniformity of composition offers an interesting 

 field for investigation. 



Petrographic Laboratory, U. S. G. S., 

 Washington, D. C, Dec. 31, '89. 



Art. XXXIV. — Additional Notes on the Tyrol ite from Utah ; 

 by W. F. Hillebrant) and E. S. Dana. 



1. Composition of Tyrolite, by W. F. Hillebrand. 



Ox a former occasion* in discussing my own and Mr. 

 Pearce's analyses of the mineral supposed to be tyrolite, I pro- 

 ceeded on the assumption that the S0 3 found by both of us 

 should be excluded a- probably derived from admixed gypsum. 

 Since then I have made another analysis on material collected 

 by myself at the Mammoth Mine, in September, 1888. So 

 pure did the material appear to be that strong hopes w T ere 

 entertained of finally settling definitely the composition of the 



* Xotes on certain rare copper minerals from Utah, this Journal, III, xxxv, 300, 

 1888. 



