426 Adventures in Scenery 



Rejuvenated landscape A previously worn-down region uplifted so as to 



renew erosive activity. 



Rhyolite A lava, usually of light color, corresponding in chemical composi- 

 tion to granite. The same molten liquid that at great depth within the 

 earth solidifies as granite would, if it flowed out upon the surface, cool 

 more quickly and crystallize less completely as rhyolite. 

 Rock Any naturally formed aggregate or mass of mineral matter, coherent 



or not, forming part of the earth's crust. 



Sandstone A sedimentary rock formed of coherent or cemented sand. 

 Schist A crystalline rock that can be readily split or cleaved because of 

 foliated structure; generally developed by shearing and re-crystallization 

 under pressure. 



Sedimentary Rocks formed by the accumulation of sediment, from water 

 (mostly) and from the air. Sediment may consist of rock fragments; 

 of the remains of animals or plants; of the product of chemical action 

 or evaporation (as salt, gypsum) ; of fragments blown from volcanoes 

 (tuffs), or mixtures of these. Sandstone, shale, limestone, coal, are 

 examples of sedimentary formations. Sedimentary deposits are generally 

 in flat beds or strata. 



Shale A fine-grained, fissile, argillaceous sedimentary rock, characterized by 

 fragile laminae or thin layers. Sometimes incorrectly called slate by 

 miners, well-drillers, etc. 



Silica An oxide of silicon (a rare non-metallic mineral known only to the 

 chemist), SiO 2 ; occurs as quartz, chalcedony, chert, flint, opal, dia- 

 tomaceous earth, and sandstone. The most abundant constituent of the 

 earth's crust. 



Silt The muddy bottoms of bays and harbors. Rock particles are inter- 

 mediate in size between the finest sand and clay. Rocks pulverized by 

 glacial ice are often spoken of as silt or rock-flour. Silt tends to crumble 

 when wet whereas clay when wet is plastic. 

 Stratum, plural strata A bed or layer of rock. 



Stone A small (or larger) piece of rock. A fragment broken from a ledge 

 (of rock) is a stone. Large natural masses of stones are generally called 

 rocks; small quarried masses or small fragments are called stones. 

 Striae Glacial markings. Straight, regular scratches, commonly parallel in 



sets, on smooth glaciated rock surfaces. 



Structural valley A relatively long narrow depression produced by move- 

 ments of the surface, as a synclinal trough. The floor of a structural 

 valley may be depressed between fault planes, on one or both sides of 

 the valley. 

 Superjacent series The nearly horizontal westward dipping strata, of late 



