THE CLARKSBUEG FORMATION. 463 



Wherever the bedded voleanics are studied the same reUitions are 

 found to exist between them and the sedimentary beds beneath and above 

 them. The volcanic formation appears to be, in general, either between the 

 Goodrich and the Michigamme formations or near the base of the latter. 

 At its eastern and its extreme western ends it seems to be a little above the 

 base of the lowermost members of the Michigamme slates, while toward 

 the center of the belt beds belonging with the Goodrich quartzites are 

 interleaved with undoubted volcanic conglomerates. The probable explana- 

 tion of these seemingly contradictory phenomena is that the rocks of the 

 eastern and western ends of the formation are a little younger than those 

 in the central portions of the belt, where the greatest volcanic activity was 

 exhibited, or, in other words, the volcanic energy first found vent in the 

 central portions of the area now occupied by the formation, and from here 

 traveled both eastward and westward. 



THICKNESS AND FOLDING. 



The thickness of the volcanic formation does not admit of accurate 

 measurement, although it must amount, in places, to several thousands of 

 feet. The indi^'idual beds in the center of the belt can not be certainly 

 separated from one another. In other places, east and west, the bedding 

 is more definite, but even here no single bed can be traced for any great 

 distance. In the western portion of the area the layers are much contorted, 

 laro-e and small folds crowding one another in an almost endless succession. 

 The strikes of these small folds point in all directions, though the prevailing 

 one seems to be toward the east and west. Generally the beds dip at high 

 angles toward the north or northeast. The series is much more highly 

 contorted than the Ishpeming formation, a result probably due to the 

 fact that a set of mixed volcanic ash beds, lavas, and sediments was less 

 resistant to pressure than the quartzites. 



PETROGRAPHICAL CHARACTER. 



A. o-eneral survey of the entire formation presents a good illustration of 

 a series of deposits formed by submarine volcanoes. The most important 

 vents of the volcanoes were near Clarksburg, though minor vents existed also 



