THE CLAKKSBITRG FOltMATION. 477 



and tuffs just descTibed; Init as this is inij)nu'ticable, they are considered as 

 a division by themselves. 



In this discussion but httle distinction is made between those rocks 

 with a congk>meratic habit and those that are more properly breccias, 

 since both contain rounded and angular frag-ments of" ])reexisting- rocks. 

 The breccias contain, in addition, angular fragments of the same compo- 

 sition as tlie matrix inclosing them. In the following pages the two terms 

 arv used indiscriminately. 



In describing the essential features of the conglomerates it is almost 

 impossible to avoid repetition of facts already stated with respect to the 

 tuffs and sediments of the formation, since the matrices of the conglomerates 

 are frequently identical with the material of these rocks. Sometimes the 

 material is sedimentary in character, sometimes tuffaceous, and sometimes 

 crystalline. The sedimentary and tuffaceous matrices do not differ in any 

 essential features from the sediments and tuffs already described, while the 

 crystalline matrices are often similar to mashed greenstones or ampliiljole- 

 schists. 



The conglomerate-schists, when viewed in the ledge, often present tne 

 appearance of a l>lack biotite-schist or hornblende-schist, containing frag- 

 ments of quartz and feldspar, of granite, of griinerite-schist, and i)f a light- 

 colored sandy rock, and yery large irregular pieces of a lustrous black rock 

 of nearly the same character as the schist itself. On the weathered surface 

 the fragments stand out plainly, but on the fresh fracture only the quartz 

 and the sandy rock become visible, the rock as a whole resembling a well- 

 crystallized " augen-schist." 



In other cases the matrix of the conglomerate is a tuff" that differs 

 very slightly, if at all, from the tuffs already described. Its fragments are 

 large ^^ieces of biotite-schist, and smaller ones of the sandy rock. These 

 latter in many instances are banded, when they are identified as fragments 

 of the sedimentary beds interstratified with the conglomerates. Grains of 

 quartz are also noticeable scattered among the feldsj)ar fragments and crys- 

 tals that help to make up the tuffaceous groundmass, and occasionally 

 fairly large pebbles of the same minerals are met with. 



A third class of the conglomerates is characterized by the sedimentary 

 nature of its o-roundmass. These rocks resemble true conglomerates and 



