ULTRA-BASIC INTRUSIVES. 221 



Hatcli^ has also described a very similar pre-Tertiary rock from Eng- 

 land as a limburgite. Kemp^ emphasizes the resemblance of the Dewitt 

 dike to limburgite, and states that it should be called limburgite.^ If we 

 attempt to extend the use of the term "limburgite" to include the pre-Tertiary 

 vitreous basalts, we shall have to include under it the rocks heretofore desig- 

 nated as picrite and picrite-porphyrite. Rosenbusch has now put the picrites 

 and picrite-porphyrites with the effusive rocks, and if of these two sets of 

 terms there is one to be discarded, it should be the name "limburgite." It 

 seems preferable imder the rules of priority to retain the name "picrite." It 

 would then seem very suitable to apply to these pre-Tertiary porphyritic 

 limburgites Hussak's old term, "picrite-porphyry," using the term "por- 

 phyry" simply with a textural significance.* 



SECTION II.— A STUDY OF A ROCK SERIES RANGING FROM 

 ROCKS OF INTERMEDIATE ACIDITY THROUGH THOSE OF 

 BASIC COMPOSITION TO ULTRA-BASIC KINDS. 



Beginning near the town of Crystal Falls, in isolated knobs, and 

 extending southeast toward the Michigamme River, where the exposures are 

 larger and better connected, there is found a series of rocks wnose charac- 

 ters are of such interest petrogenetically as to warrant a detail description 

 of them. 



These rocks are all intrusive in character, with few exceptions are 

 medium to coarse grained, and, while the granitic texture is predominant, 

 there are certain facies in which the texture is porphyritic and even parallel. 

 They have been only slightly affected by dynamic action, and these cases 

 are purely local. Analyses show them to vary in chemical composition 

 from those of intermediate acidity to those of ultra-basic character. 



The prevailing rocks are, on the one hand, diorites of intermediate 

 acidity ranging to more acid rocks, tonalites, quartz-mica-diorites, and 



rocks, representing as it probably does the olivine-free form of the limburgite (augitite). Geology of 

 Angel Island, by F. L. Ransome : Bull. Geol. Dept. Univ. of California, Vol. 1, 1894, p. 200. 



' The Lower Carboniferous volcanic rocks of East Lowthian (Carlton Hills), by F. Hatch : Trans. 

 Royal Acad. Edinburgh, Vol. XXXVII, 1892, p. 115. 



"Op. cit.,p.460. 



'"'Taking plutonio rocks as practically the granitoid, and volcanic as the porphyritic, the 

 Dewitt rock is a basaltic dikeof the same composition and texture as limburgite, and should be called 

 limburgite, even if it is not a surface flow." (Loc. cit., p. 460. ) 



-•I believe E. Hussak was the first to use this term for a somewhat similar rock. Pikrit-phorphyr 

 von Steierdorf im Banat, by E. Hussak : Verhaudl. K.-k. geol. Reichsanstalt, 1881, pp. 258-262. 



