PYEOCLASTICS OF HEMLOCK FOEMATION. 137 



in water, and whose fragments show a rounded character. Should the 

 fragments be angular, the rocks may be called "volcanic breccias." 



It has been found practicable to maintain this distinction in earlier 

 studies on Tertiary volcauics,^ and it is also maintained in the present study 

 of pre-Cambrian volcanics. I am confident the same distinction could be 

 made more generally than it is, and would in that case tend to a greater 

 precision in the separation of rocks of different characters. However, it is 

 rather difficult to separate true eolian deposits of volcanic fragmentary 

 materials from those in which the fragments have been deposited rapidly 

 through water without having embedded organic remains and without 

 having undergone sufficient attrition to be much rounded. More or less 

 rounding, it is well imderstood, results from the attrition of the volcanic 

 ejectamenta during their ascent and descent through the air, so that they 

 may in this respect resemble many of the sedimentaries. The exact mode 

 of origin of many of the volcanic fragmental deposits of the Michigamme 

 district is not clear. The greater portion appear to be of true eolian origin, 

 and where the origin of any is in doubt it has been put with those of eolian 

 oi'igin. 



COARSE TUFFS. 



The coarse tuff's include rocks composed of fragments of all sizes, from 

 the large volcanic blocks to the fine-grained particles of sand and dust 

 which fill in the interstices. The ejectamenta may be more or less rounded 

 by attrition during their progress through the air, so that if a refinement of 

 the nomenclature should be needed one might very properly be justified in 

 speaking of tuff breccias and tuff conglomerates. 



Tuffs are very common and characteristic for the district. The char- 

 acters of the beds is best shown on the weathered surfaces. Here the 

 scoriaceous and dense light-green fragments stand out well from the 

 brownish-red matrix of more altered, finer fragments and cement. On a 

 fresh surface the interstitial material usually has a darker green color than 

 the fragments. The fi-agments have a prevailing green color, but many, 

 especially in sections, are brown, much darker than any of the rocks 

 forming the lava flows. The larger fragments are usually sharply angular, 

 but in m.any cases are more or less rounded because of attrition during 



1 Die Gesteine des Dnppauer Gebirges in Nord-Bobmen, by J. Morgan Clements : Jahrbuch K.-k. 

 geol. Eeichsanstalt, Vol. SL, 1890, p. 324. 



