Handbook of Paleontology 75 



a large amount of mica, a fine, scaly, silky variety of 

 muscovite known as sericite. Due to crustal movements 

 the surface is flat or sometimes curved, folded or 

 crumpled. Phyllites may have* been originally sedimen- 

 tary rocks or igneous rocks which have in the process of 

 metamorphism been subjected to pressure and great 

 shearing. Marble is a metamorphic product of sedimen- 

 tary rocks composed of carbonate of lime, such as lime- 

 stone, chalk etc. When the limestones are dolomitic, 

 dolomite marbles are produced; pure carbonate of lime 

 produces lime marbles. Marble is a crystalline granular 

 rock composed of grains of calcite, and unlike most meta- 

 morphic rocks is massive, showing no signs of schistose 

 cleavage or schistosity. The normal color of marble is 

 white but the presence of impurities produces gray, yel- 

 low, red or black colors. The great deposits of marble 

 used for constructional purposes are the result of regional 

 metamorphism. 



Metamorphic rocks, too, are characterized by folding 

 and faulting. Their age is that of the rocks from which 

 they were derived, and is sometimes difficult, almost im- 

 possible, to determine. Fossils, if still present, are a help, 

 but more often than not they are unrecognizable or have 

 completely vanished and age can then be determined only 

 by correlation with rocks whose age is already known. 



CONDITIONS OF LIFE IN THE SEA TODAY 



Most of the formations that will be treated in this 

 handbook are of marine origin — deposits laid down in 

 the sea. In order to have a proper understanding of 

 these geologic formations and the fauna they carry, 

 it is well to know something of the conditions of life 

 in the sea today and particularly of the animal associa- 



*r 



