432 EEPORT ON NATIONAL MUSEUM, 1886 



augite trachytes, according as either of these accessory minerals pre- 

 dominates. 



Phonolites differ from trachytes in carrying one or both of the min- 

 erals nepheline or leucite in addition to the other constituents named. 

 They bear the same relations then to the trachytes as do the elaeolite 

 syenites to the syenites proper. 



Neither trachytes nor phonolites are, so far as now known, common 

 rocks in the United States. Zirkel * describes numerous trachytes 

 from the areas covered by the Fortieth Parallel survey, and Caswell t de- 

 scribes both trachytes and phonolites from the Black Hills, Dakota. 

 Recent investigations by Wadsworth J and Messrs. Hague and Iddiugs§ 

 show, however, that the supposed trachytes of Zirkel were in large part 

 if not altogether andesites, and it is very probable that similar tests 

 applied to many other cases heretofore described would be productive 

 of similar results. However this may be, the utility of the rocks in 

 America is purely prospective. 



Their colors and textures are such that they can never be used for 

 other purposes than rough construction, as is the case with the majority 

 of the younger eruptives. 



I.— AUGITE (ENSTATITE, HYPEESTHENE) PLAGIOCLASE 



ROCKS. 



(1) DIABASE. 



(Diabase, from the Greek word otapjuat^^ to pass over; so called be- 

 cause the rock passes by imperceptible gradations iuto diorite.) 



The diabases are entirely crystalline granular rocks, composed essen- 

 tially of plagioclase feldspar and augite, with nearly always magnetite 

 and frequently olivine. Geologically they are i^re-Tertiary eruptive 

 rocks, basic in composition, occurring in dikes, intruded sheets, and 

 lava flows. Their mode of occurrence is quite similar to that of basalt, 

 from which they differ chiefly in date of eruption and the amount of 

 alteration they have undergone. In structure they are as a rule mas- 

 sive, but schistose varieties occur and more rarely spherulitic forms. 

 The texture is as a rule fine, compact, and homogeneous, though some- 

 times porphyritic or amygdaloidal. The colors are somber, varying 

 from greenish through dark gray to nearly black, or sometimes black 

 when freshly quarried, but becoming greenish on drying. || 



* Micro. Petrography, fortieth parallel. 



t Geol. Black Hills of Dakota. 



tProc. Bos. Soc. Nat. Hist,, Vol. xxi, 1881, p. 243, and Vol. xxn, 1883, p. 412. 



$ Am. Jour. Sci., Vol. xxvn, 1884, p. 453. 



|| Mr. J. P. Iddings suggests that the change in color from dark, blue hlack, and 

 greenish, as noticed in diabase of New Jersey, is clue to the drying of the serpentine or 

 chlorite, which results from the alteration of the included olivine. (Am. Jour. Sci., 

 May, 1886, p. 330.) 



