886 A TREATISE ON METAMORPHISM. 



gain hornblende in consequence of the exomorphic effect, and in that way 

 produce a graywacke-schist containing much hornblende and very similar 

 to hornblendic-schists of igneous origin. (See PI. X, D.) 



The graywacke-slates, graywacke-schists, and gravwacke-gneisses, of 

 course, grade into the graywackes from which they are derived. Further, 

 they grade on the one hand into the arkose- schists and arkose-gneisses, and 

 on the other hand into the pelite-schists and pelite-gneisses. 



PELITE ORDER. 



MUD FAMILY. 



Mud is the finest of the mechanical sediments. The greatest deposits 

 of mud form at the mouths of large rivers. Where very great rivers empty 

 into the sea deltas of immense thickness and size are built up of mud, as, 

 for instance, the delta of the Mississippi. Great deposits of muds are also 

 forming in the estuaries into which the rivers enter, such as the estuaries of 

 the Atlantic. These delta and estuary deposits, besides being extensive, are 

 of immense thickness. In mediterranean seas great deposits of mud may 

 be built up as the products of many rivers. Thus Willis explains the Ham- 

 ilton shale of the Devonian of the Appalachian region, a formation extending 

 from New York to Maryland, and having a thickness of from 350 to 420 

 meters." Also in the open sea, as a result of the process of sorting, the 

 muds are carried beyond the psammites, and so extensive formations of 

 mud of moderate thickness are built up. 



Muds deposited in mediterranean seas or in the open ocean very fre- 

 quently are interstratified with or grade into the psammites toward the 

 shore. Seaward they frequently are interstratified, intermingled, or grade 

 into the calcareous deposits. To a less extent similar transitions are found 

 associated with delta and estuary deposits. Consequently we have deposits 

 intermediate between muds and sandstones, and between muds and lime- 

 stones. From the foregoing it follows that muds vary in composition from 

 that of the various psammites to that of the carbonates. The rocks inter- 

 mediate between the muds and the carbonates usually contain a large 

 percentage of calcium, magnesium, and carbon dioxide. 



On the average the material of which mud is composed has been 



a Willis, Bailey, Paleozoic Appalachia, or the history of Maryland during Paleozoic time: Mary- 

 land Geol. Survey, vol. 4, pt. 1, 1900, pp. 57-61. 



