PYROCLASTIC ROCKS 525 



way. The aquatic forms found comprise a thin-shelled Planorbis, 

 always occurring in a crushed condition, a Physa and a single speci- 

 men of a bivalve, and eight species of fishes. Birds' feathers and 

 bones and the skeletons and feathers of entire birds have also been 

 found. Strongly sun-cracked layers ]joint to repeated desiccation 

 of the shallow lake, and this condition, first recognized by Lesque- 

 reux, is not excluded by the character of the fish, the water plants, 

 water insects or molluscs. ( Scudder-22 ; 23.) 



Volcanic debris falling on the surface of the land will show 

 little regularity of deposition. The coarser material will exhibit 

 no bedding lines or only the very rudest ones, while volcanic sand 

 or ashes may not infrequently show a moderate stratification, which 

 will generally be more marked in the finer material. Where the 

 ashes come to rest on a slope these stratification planes will be in- 

 clined. 



Organic Remains of Modern Pyroclastics. Organic remains are 

 not infrequent in pyroclastic rocks. They are best preserved in the 

 finer grained deposits. Of these the remains of man and his works 

 are often the most al)undant in the modern tufifs, whole cities and 

 their inhabitants having not infrequently been buried. At Hercu- 

 laneum and Pompeii human bodies were completely encased in the 

 fine mud from Vesuvius, which on hardening formed a perfect 

 mold of the body, so that plaster casts could afterward be made 

 from them. 



Older Pyroclastic Deposits. From South America and from 

 portions of the Great Plains of western North America, Tertiary 

 reolian tufifs (Anemopyrolutytes) rich in the bones of mammals, 

 have been described. In the Eocenic deposits of the Wind River 

 Basin in Wyoming, a white tufif bed 13 feet in thickness lies 

 interbedded with the Wind River sands and clays. (Sinclair and 

 Granger-28 :o ? et seq.) It rests on a heavy brown sandstone con- 

 taining rolled pebbles of tufif and is succeeded by greenish shales. 

 In the upper 4 or 5 feet of tufit" occurs much pumice ; snow white 

 grains from the size of bird-shot to small peas are found enclosed 

 in a fine-grained gray matrix. The most abundant macroscopic 

 mineral enclosed in the white pumice is biotite, while small frag- 

 ments of feldspar are occasionally found. Under the microscope the 

 fine felt-like mass of volcanic glass of the pumice encloses in the or- 

 der of relative abundance: (i) orthoclase, often zonal, sometimes 

 twinned, either with recognizable crystallographic boundaries, more 

 or less broken, or in small laths; (2) plagioclase ; (3) olive green 

 biotite; (4) hornblende; and, (5) black opaque grains, probably 

 iron oxide. 



