THE CLAKKSBURG FORMATION. 473 



almost exclusively of tuffaceous material there exists a larg'e series of inter- 

 mediate rocks that ai'e mixtures of sedimentary and pyroclastic material iu 

 varying proportions. On the one hand they pass direct!}- into widl-defined 

 sedimentaries, and on tlie other hand into t\'])ical tuffs. In macroscopic 

 appearance they resemble the amphiljole-ljearing- elastics descrilxwl in the 

 last paragraph. 



In thin section the gradation varieties diB'er from tiie liornMemlic frag- 

 mental rocks sim})ly in the absence of any great quantity of (juai-tz grains. 

 Only occasionally is the tuffaceous character of some of their constituents 

 revealed by their structure. Usually the rocks have Ix^en recrystallized, so 

 that their original components liave entirely disappeared. No\\' and then 

 a quartz grain is observable in the midst of an aggregate of green horn- 

 blende and brown biotite, but the characteristic clastic structure, as well as 

 the characteristic tuffaceous structure, is wanting. 



The tuffs. — The tuffs, like the mixed tuffs and sediments, are usually 

 largely recrystallized. Quite frequently, however, beds of them are found 

 in which the tuffaceous structure has been well preserved. In the hand 

 specimens these rocks, where fresh, present the usual aspect of a hornbleudic 

 tuff; where altered they closely resemble the "Schalsteins." In almost all 

 of them more or less calcite may be observed. In a few, as, for instance, 

 in the rock at the crossing of the Chicago and Northwestern and the 

 Milwaukee and Northern railroads, just east of Champion, the calcite is 

 present in such large quantities that the i-ock becomes practicalK' a lime- 

 stone. Even in these much altered forms, however, the tuffiiceous character 

 of the beds may often be recognized in the hand specimen, as fragments of 

 minerals and rocks of all shapes may be seen crowding the calcareous mass 

 in which they lie. Many of the fragments have been completely altered to 

 calcite, which nevertheless has preserved their outlines by forming from 

 them perfect pseudomorphs. 



The less altered tuffs are more interesting from a petrographical stand- 

 point, since they often present excellent proofs of their original character. 

 In general the fine-grained, pure tuffs are more frequently found interbedded 

 with volcanic conglomerates and breccias than with sedimentary lavers, 

 although alternations of the tuffs with the sediments are not uncommon. 



