PYROCLASTICS AND AUTOCLASTICS 291 



Volcanic sand is characterized by irregular and sharply angular 

 outline, giving no evidence of erosion except in the case of the 

 larger particles, where partial rounding from mutual attrition dur- 

 ing suspension occurs. The sands are comparatively well sorted, 

 according to size, the finest material being often carried far 

 away. More or less well defined crystals are generally visible un- 

 der the microscope, but as a rule much of the material is amorphous, 

 showing flow structure or vesicular character. (26 : di'p, with refer- 

 ences.) Subsequent modification and rounding by water or air 

 gives us hydropyroclastics and anemopyroclastics, while weathering 

 produces atmopyrodastics. When water laid, remains of marine or 

 fresh-water organisms may be enclosed in these strata, or drifted 

 land organisms may be entombed, as in the case of the Tertiary 

 "Lake beds" of Florissant, Colorado. 



2. The Autoclastics. This group comprises all rocks shat- 

 tered or crushed within the earth either by pressure of one 

 mass upon the other, or by movement of rocks over each 

 other. Fault-breccias and the material of the "crush zones" 

 must be classed here, as well as fragments produced by 

 avalanches. Earthquake-shattered rocks may also be in- 

 cluded, though they may likewise be considered transitional 

 to the pyroclastics. By far the most important autoclastic 

 products, however, are those resulting from glacial erosion. 

 Ice, including all the material frozen in it, is a part of the 

 earth's crust while it exists, and hence any material ground 

 up between the ice and the rock on which it moves is of the 

 type of the material crushed between other moving rock 

 masses (ex. fault breccia). Furthermore, since all ice-trans- 

 ported material has received its most characteristic features 

 from that agent, we may with propriety include such material 

 in this group, even though it w^as originally broken by atmos- 

 pheric agencies. Such rocks, if determinable, would come 

 under the compound heading, auto-atmoclastic. 



Fault breccias or autorudytes partake of the composition of the 

 rocks from which they were formed, with probably slight changes 

 due to secondary modifications. In limestones there may thus occur 

 pure autocalcirudytes, while among pure quartz rocks autosilici- 

 rudytcs may occur. In general, however, the composition of auto- 

 clastic rocks is quite impure, and this is particularly the case in the 

 autorudytes and other types which result from the consolidation of 

 glacial deposits. The latter are of the most importance to the 

 stratigrapher, for there can be little doubt that they are represented 



