Handbook of Paleontology 15 



INTRODUCTION 



THE ROCKS 



Geologically the term rock refers to the material com- 

 posing the earth's solid shell. Thus defined it includes the 

 solid or bed rock and the unconsolidated or mantle rock 

 which is derived directly from the solid rock beneath 

 through disintegration and decomposition and grades 

 gradually into it, as is the case with the residual soils of 

 our southern states, or which has been transported 

 through some agency and rests abruptly upon it, as in the 

 case of glacial deposits, stream deposits etc. It is the 

 solid rock which will be considered here. According to 

 their mode of origin rocks fall into three natural groups : 

 sedimentary, igneous and metamorphic. The sedimen- 

 tary rocks include bedded rocks formed by the deposition 

 of mechanical sediments through the agency of water or 

 the air, as sandstones, shales etc. Here also belong chem- 

 ical precipitates and evaporation products from solution 

 in water, as cave deposits (stalactites, stalagmites), rock 

 salt, gypsum etc. and deposits formed through organic 

 agencies in the air or water, as coral and shell limestones, 

 chalk, coal, guano etc. Igneous rocks (Latin igneus, 

 fiery), are rocks, such as granite, usually crystalline and 

 composed of a number of minerals and formed by the 

 solidification of molten material. Metamorphic rocks 

 (Greek meta, over; morphe, form), are known as sec- 

 ondary rocks because they are derived from rocks of the 

 two preceding groups by certain processes that bring 

 about recrystallization of the rock constituents either with 

 or without alteration of the chemical composition of the 

 mass, as in the formation of gneisses from granites, 

 marbles from limestones, schists from sandstones etc. 



