326 University of California Publications in Geology [Vol. 7 



' ' Great masses of angular blocks of all sizes accumulate at the foot 

 of cliffs and on mountain slopes as talus, which shows an imperfect 

 division into layers and is slowly but continually creeping downward. 

 By the deposition of some cementing material (usually CaCO,) in the 

 interstices of the talus the blocks may be bound into a solid mass, called 

 breccia, of which the peculiarity is that the fragments composing it are 

 angular, not rounded." 



"In a cave it frequently happens that angular fragments fall from 

 the roof and are cemented into breccia by deposits of stalagmite." 



The diversity of use to which the word breccia is put by this 

 author is not peculiar to his text. Tt is equally well exemplified 

 in other books, some authors emphasizing - one use of the word 

 and some another, but nearly all using it in more than one sense. 

 The term is, indeed, applied to other things than are indicated 

 by the foregoing quotations. For example, the aggregate of 

 blocks of rock imbedded in lava due to the breaking up of a 

 lava crust while the flow beneath is still molten, is commonly 

 called a breccia; and certain gangues of ore that have been broken 

 and recemented, such as the zinc ores of the Joplin district, are 

 similarly designated. 



It is evident from this current usage that breccia is a generic 

 term applicable to a rock of any kind which is composed of an 

 aggregate of angular fragments, the origin of the fragments and 

 their mode of aggregation being ignored except by the intro- 

 duction of a supplementary word or phrase. The usefulness of 

 the term inheres in that comprehensiveness which is character- 

 istic of the terminology of an immature science. 



Alluvial Fans are Formations of Vast Extent. — In arid 

 regions of bold relief there are vast deposits of "wash" or rock 

 detritus in the form of confluent or interdigitating alluvial fans, 

 which are among the most important continental formations now 

 in process of accumulation. Probably more than half of the area 

 of the state of Nevada is occupied by such deposits and in large 

 parts of Utah, New Mexico, Arizona, California, and Mexico, they 

 are the most extensive formations encountered by the geologist. 

 These alluvial fans consist of rock detritus which grades in size 

 from coarse blocks near the mountains to fine silt in the playas 

 or river bottoms. The assortment of materials is effected by the 

 diminishing capacity and carrying power of running water in 



