THE CLARKSBURG FOR.^IATION. 481 



formatiou. This is unquestionably a set of sediments, tuffs, lavas, and 

 volcanic and sedimentary conglomerates that were deposited beneath the 

 surface of some bod)^ of water. The volcanic contributions to the series 

 pi'obably exceeded in volume those contributed by aqueous agencies, 

 although these latter were by no means small in amount. Of the volcanic 

 contributions the larger portion was in the form of volcanic cinders, ashes, 

 etc., a much smaller portion being in the form of lavas. This fact would 

 indicate that the eruptions were violent, like the type represented by 

 Vesuvius at present, rather than quiet, like the Hawaiian volcanoes. That 

 they were intermittent is proved by the numerous alternations of tuffs with 

 sedimentary layers. The conglomerates that occur in the formation are 

 simply tuffs or sediments containing large fragments of preexisting rocks, 

 sometimes waterworn and sometimes angular. The former were worn from 

 rocks that were exposed to the action of the waves when the deposits in 

 which they are found were being laid down. The latter Avere torn from 

 the throat of volcanic vents or were produced by the shattering of rock 

 beds already existing, or perhaps, in the case of some tuff fragments, by 

 the breaking of the rock beds actually in process of formation at the 

 time when the conglomerates containing them were being built up. 



All forms of volcanic products are recognized among the beds compris- 

 ing the series except volcanic bombs and perhaps those peculiar breccias 

 produced by the breaking of a lava's crust and the cementing of the 

 fragments thus formed into a solid rock by the cooling of the liquid mass 

 in which they became embedded. The bombs may possibly be represented 

 by some of the altered greenstone bowlders occasionally met with in the con- 

 glomerates, and the lava breccias may be represented by some of the scliis- 

 tose conglomerates in which irregularly shaped schistose fi-agments are 

 embedded. If so, however, there are no positive proofs of the facts. 



INTERESTING LOCALITIES. 



Good exposures of the rocks of the Clarksburg series are found along 

 the north and west lines of the NW. i sec. 4, T. 47 N., E. 29 W. (Atlas Sheet 

 XIII), near Champion. The hill immediately north of the west quarter post 

 of the section is made up in large part of well-ljedded conglomeratic rocks 

 with a green schistose matrix. The beds strike a little north of west and dip 



MON XXVIII 31 



