THE CLARKSBUEG FORMATION. 485 



The bosses of gTeenstone mark the sites of the old \'oloanic vents from 

 which the uiaterials of the tuffs were erupted. Tluj most prominent of these 

 were situated a few miles to the southeast of the village of Clarksburg, 

 though others were opened from time to time to the eastward and the west- 

 ward of this center. 



The ashes and lavas sent from these vents fell into water and were 

 interbedded with sediments. The ])yroclastic material Ijecame consolidated 

 into tuffs and the sediments modified into slates, schists, and graywackes. 

 The former mark the periods of volcanic activity and the latter mark periods 

 of rest. Mixed tuffs and sediments were formed during the less violent stages 

 of the eruptions. After deposition the beds were hardened by alteration and 

 by the formation of new products resulting from the decomposition of the 

 constituents already existing- in the beds, with the addition, perhaps, from 

 extraneous sources, of a little quartz. 



The conglomerates and breccias interstratified with the sechments and 

 tuifs are simply these rocks with the addition to them of bowlders and frag- 

 ments, mainly of preexisting rocks cast out through the volcanic vents or 

 broken from ledges by the action of the wa\'es, but occasionally of jjor- 

 tlons of lavas and tuffs shattered in the process of solidifying. In the latter 

 case the fragments are very similar to the rock masses in which they are 

 embedded. 



The lavas associated with the fragmental rocks are rare. They consist 

 of altered diabasic or basaltic amygdaloids that have lost nearly all of their 

 original stinictural features. 



All the evidence obtained through the microscope confirms the conclu- 

 sion of the field study, ^•Iz: That the Clarksburg series consists of an 

 accumulation of the ordinaiy deposits of Ishpeming and Michigamme 

 sediments, with interbedded pyroclastic material erupted by a volcano whose 

 principal vents are located by the greenstone knobs in the vicinit}' of 

 Clarksburg village. 



In their present forms the greenstones are altered diabasic porphy rites; 

 the lavas, basalts or diabases; the pure sediments, graywackes or slates; 

 and the tufts, "Schalsteins," where much weathered, and where but slightly 

 weathered, aggregates of amphibole, biotite, altered plagioclase, magnetite, 



