480 TUE MAKQUETTE IKON-BE A KING DISTEICT. 



crystals lie in all azimuths, their longer axes cutting the plane of schis- 

 tosity. The contact effects were probably the result not so much of the 

 heat alone to which the beds Avere subjected as to the hot solutions that 

 passed between the basic tuffaceous beds and the acid sedimentary ones, 

 and between the basic and acid components of the mixed beds. 



The conglomerates with a tuffaceous grouudmass present in the thin 

 section nearly the same appearance as the hornblendic bands of the sedi- 

 mentary rocks described in the last few paragraphs. Very rarely is a typical 

 tuffaceous structure observable, although on the sides of the weathered 

 ledges this structure is very plain. Biotite is present in small quantity 

 only, while green hornblende is abundant. In nearly all cases some sedi- 

 mentary material can be detected as a fine-grained, almost dusty aggregate 

 between the lai-ge aniphibole grains, but in no case is it in any large 

 quantity. In these conglomerates the most interesting- fragments are those 

 that are sinjilar to the sedimentary rocks of the formation. Many of them 

 are large, Avhite, rounded pebbles, which in thin section are found to possess 

 a well-preserved fragmental structure. They consist of quartz and altered 

 feldspar grains, the former predominating, sericite and biotite in veiy small 

 quantities, and magnetite in dust grains. Scattered here and there through 

 the mass are delicate plumose groups of green hornblende that are cA-idently 

 much vounger than the clastic grains. In cross-section the amphiboles are 

 idiomorphic. In all its essential features the rock of these pebbles is iden- 

 tical with that of the sedimentary beds interstratified with the tuff's, even 

 to the presence in it of the introduced idiomorphic amphiboles. The other 

 fragments oceuri-ing in these rocks need no description. 



The only other class of conglomerates distinguished is that in which 

 the rocks possess a sedijnentary groundmass. This is composed of a frag- 

 mental aggregate of quartz and a little feldspai', large quantities of brown 

 biotite, a small (pxantity of magnetite, and the usual spicules and crystal- 

 loids of the green amphibole, occurring sometimes in single grains and 

 sometimes in plumose or sheaf-like bundles. 



COXCLUSIONS. 



In the petrographical study of the rocks we find abundant confirmation 

 of the accuracy of our conclusions regarding the origin of the Clarksburg 



