136 THE CEYSTAL FALLS IRON-BEAEING DISTRICT. 



also of igneous origin. In au eruptive breccia the fragments may be like 

 or unlike. Likewise the matrix may be like or unlike the fragments. 

 Where the fragments ha^-e been rounded during the movement of the erup- 

 tive magma surrounding them, the resulting rock may be called an eruptive 

 pseudo-congJomcyate. 



Eruptive breccias are not very common in the Crystal Falls district. 

 Where they do occur, the fragments, while predominantly angular, are to 

 some extent more or less rounded, and are similar in natui-e to the matrix 

 in which they lie. Since the rocks which form them preserve the main 

 characters of the massive lava flows which have just been described, they 

 will not be discussed in detail. The exact method of the formation of these 

 breccias could not be told. 



In one case, in which both fragments and matrix are amygdaloidal, 

 it appears probable that the occurrence represents a true flow breccia in 

 which the broken surface of a lava flow had been recemented by a later 

 lava flow of the same kind of rock, or that it represents a very possible 

 case in which the lava welled up through and flowed over portions of its 

 own crust, cementing the fragments. In such breccias a flow structure 

 around the fragments is quite plainly shown and the matrix possesses a 

 peculiar ropy appearance. In one instance, in which both the fragments 

 and matrix were macroscopically nonamygdaloidal, it is probable that they 

 were formed under considerable pressure, and that this was a case in which 

 lava was forced up through a previously consolidated mass of rock of like 

 character, and in its passage carried with it various fragments, forming 

 an eruptive ^^reibungs-hreccia^' ov friction breccia. 



VOLCANIC SEDIMENTARY EOCKS. 



Under the term "tuffs" have been very generally included all kinds of 

 volcanic clastic rocks.^ This is probably due to the fact that there is fre- 

 quently considerable difficulty in discriminating between eolian deposits 

 and those which have been deposited in water. It seems desirable, wherever 

 it is possible, to make this discrimination. To that end I shall in the fol- 

 lowing pages restrict the term "tuff'" to eolian deposits. The term "volcanic 

 conglomerate," or, for the sake of brevity, simply "conglomerate," will be 

 used for those coarse deposits whicli have been sorted by and deposited 



' Text-book of Geology, by Sir Archibald Geikie : 3d ed., p. 135. 



