Handbook of Paleontology 35 



is a coarse-grained sandstone with more or less angular 

 particles and a silicious cement, also known as "millstone 

 grit" because of its use for grindstones and millstones. 

 Arkose is a sandstone in which there is much feldspar 

 present and often considerable mica, and when firmly 

 cemented bears a small resemblance to granite at a casual 

 glance. The particles are irregular and angular. The 

 character of the particles and the composition of the rock 

 indicates a derivation from disintegrating granite and 

 deposition only a short distance from the place of origin. 

 Arkosic sandstones grade into conglomerates and brec- 

 cias. Green sandstone is quartz sandstone or arkosic 

 sandstone rich in glauconite grains which gives a green 

 color to the rock, also called glanconitic sandstone. Rocks 

 of this kind are sometimes very loosely consolidated, fri- 

 able, and they are then known as greensand and green- 

 sand-marl. Freestone is a quartz sandstone in which the 

 grains are loosely consolidated, commonly only bound 

 together by pressure. This is used as a building stone. 

 Graywacke is sandstone rock of prevailing gray color, 

 an impure, highly argillaceous sandstone which is vari- 

 able in composition, texture and structure. Like arkoses 

 these rocks have quartz and feldspar ; but in addition 

 there are angular or rounded bits of various kinds of 

 rocks — shales, slates, granites, basalts etc. — and when 

 these pieces are larger the rocks pass into conglomerates. 

 When the grains composing a sandstone are wholly or 

 largely composed of carbonate of lime, as the sands 

 around coral reefs today, the rock is known as lime sand- 

 stone. Such sandstones are the basis of many of the lime- 

 stones of various geological formations. Wind blown 

 sand deposits form eolian sandstones which are recog- 

 nized by their structure and the character of the grains. 



