THE CLARKSBURJ; FOllMATION. 475 



ejnbedded in a faiutl}- polarizing aggregate that is apparently made up of 

 quartz and plagioclase in very tine grains, like the aggregate produced by 

 the devitrification of a glass. Under high powers, in addition to the mag- 

 netite grains there are detected certain small purplish plates resembling 

 those formerly so well known under the name of " Eisenglimmer." The 

 magnetite of the fragments is nontitaniferous, while that in the inclosing 

 rock is often strongly titaniferous, if we may judge from the great quan- 

 tities of leucoxene and sphene in the latter and its absence from the former 



In the above descriptions detailed reference is made only to the sedi- 

 ments and to the tuffs, as though these rocks were the move, important 

 members of the Clarksburg formation. As a matter of fact, the well-defined 

 sediments and the typical tufts constitute a much smaller aggregate in the 

 series than do the mixed sediments and tufts. They are described in some 

 detail because they have retained their original characteristics better than 

 have the mixed rocks, and so aff"ord better evidence as to the nature of the 

 formation than do 'the latter. The variet}' of the mixed rocks is great. 

 They well deserve close stud}-; but to describe them in detail would 

 unduly enlarge this monograph. It is enough for the present to reiterate 

 the statement that the mixed rocks are intermediate in their characters 

 between the types of rocks that are above described. 



From the facts already related it is clear that we have in the Clarks- 

 burg formation a series of typical tuffs, together with a series of mixed 

 sediments and tufts, formed by the accumulation of volcanic dust and ashes 

 in a basin in other [nirts of which the deposition of ordinary land debris 

 was going on. The variation of the quantities of quartz, amphibole, and 

 biotite in the alternate beds is easily accounted for in the safe assumjition 

 of a variation in the volcanic activity. The alteration and crystallization of 

 the beds are ascribed partly to contact action and partly to mashing. The 

 former was due, no doubt, more to the chemical effects of the solutions 

 passing between lavas, tufts, and sediments than to heat alone. The 

 mashing resulted in the contortion of all the beds of the formation. 



THE HORNBLENl>E-SCHISTS. 



The processes which changed the mixed sediments and tuffs of the 

 Clarksburg formation nuist have been very similar to those which produced 



