1 913 ] Lawson: Designation of Alluvial Fan Formations 331 



It is to be noted here that cliff talus is to be discriminated 

 from alluvial fan accumulations. A typical talus or scree, 

 whether cemented or not should not be included under the term 

 used to designate fan deposits. The latter are transported and 

 laid down by running water and are therefore sedimentary. The 

 former accumulate under the direct action of gravity and are 

 not sedimentary in their mode of deposition. Slight admixtures 

 of talus may enter into the composition of certain fans on their 

 upper margins, but this fact will not ordinarily affect the dis- 

 crimination. Should it be found expedient to refer to talus 

 formations by a specific petrographieal designation, as it well 

 may be in a complete account of detrital rocks, it is desirable 

 that the term should be distinct from that applied to fan deposits. 



Toward its lower limit of coarseness fanglomerate grades into 

 what is ordinarily termed arkose. This arkose is far from uni- 

 form in size of grain, however, and if we set arbitrarily 5mm. 

 as the maximum diameter of fragments in arkose, pieces of this 

 and smaller sizes will be found intimately mixed with much finer 

 material. The same defect in sorting which characterizes the 

 fanglomerate is also exemplified in the arkose. 



On the lower slopes of the alluvial fans the arkose passes into 

 silt and mud which also occupy the bottoms of the playas and 

 river flood plains. 



In these alluvial embankments, which form such notable con- 

 tinental deposits of sedimentary origin, we have thus three rock 

 types, viz : fanglomerate, arkose and silt. The first two of these 

 may be easily discriminated from marine deposits, and it seems 

 probable that microscopic methods of study will supply the 

 criteria for distinguishing the third, in eases where this cannot 

 be done on the basis of field associations. 



Varieties of Fanglomerate. — Fanglomeratcs may vary greatly 

 in the character of the materials of which they are composed. 

 The most typical embankments are composed of fragments of 

 variously indurated sedimentary rocks of mountain ranges under- 

 going rapid degradation in an arid climate. Some of these ranges 

 may, however, be made up chiefly of igneous rocks, either 

 plutonic or volcanic and the composition of the flanking fan- 

 glomerate will correspond to the character of the detritus sup- 



