OOEEELATION OF UPPER HUKONIAN SERIES. 165 



niiqiiestionably the western continuation of the Michigamme formation, to' 

 Avhich the rocks correspond petrograpliically. The Michigamme formation 

 has recently been carefully studied by Van Hise, and described by him in 

 detail in Monograph XXVIII (Chap. IV). A less detailed description is 

 given in the Fifteenth Annual Report (pp. 598-604). Since such great 

 petrographical similarity between the Upper Huronian deposits in the west- 

 ei'nhalf of the Crystal Falls district and the above formation in the adjoining 

 districts exists, and since nothing of exceptional interest has been observed 

 in their study, the reader is referred to the articles mentioned for details. 

 The following general description, while based upon the study of many 

 exposures, specimens, and 75 sections of these Crystal Falls rocks, may still 

 be considered to some extent as an abstract of the above articles in which 

 the few changes made necessary by the slightl}- different characters have 

 been incorporated. 



PETROGRAPHICAI. CHARACTERS. 



From the above general statement it is seen that the Upper Huronian 

 comprises rocks both of sedimentary and of igneous origin. 



The preponderant deposits of the western half of the Crystal Falls 

 district were muds and grits. With these were subordinate quantities of 

 carbonates. In a few places sheets of basic rocks were intruded between 

 the sedimentary beds and are now found alternating' with them. Widely 

 distributed basal conglomerates, coarse quartzitic conglomerates, and 

 quartzites, such as characterize the lowest horizon (the Groodrich quartzite) 

 of the Upper Huronian of the Marquette district, are absent. Work already 

 completed outside of the immediate area covered by this report shows the 

 presence of a small area of surface volcanics associated with the inodified 

 Upper Huronian sediments. This evidence of contemporaneous A^olcanic 

 acti^aty is closelj" paralleled by the Clarksburg volcanics of the Upper 

 Marquette of the adjoining district.^ 



SEDIMENTARY ROCKS. 



The sedimentary rocks of the Upj^er Huronian series in the western 

 part of the Crystal Falls district are graywackes, ferruginous gray wackes ; 

 micaceous, carbonaceous, and ferruginous clay slates and their crystalline 



'Fifteenth Ann. Rept. U. S. Geol. Survey, cit., pp. 604-607; Monograph U. S. Geo]. Survey, Vol. 

 XXVIII, cit., pp. 460-486. 



