62 New York State Museum 



through erosion, be represented only by isolated 

 patches miles apart. Sometimes a bed of rock varies 

 greatly even in a short distance in its lithological 

 character, that is, in the mineral composition and tex- 

 ture as well as in external appearance. These are 

 some of the difficulties that face the geologist, but in 

 all these cases fossils when present are usually a key 

 to definite knowledge of the age of the strata. 



The actual age of rocks expressed in thousands, hun- 

 dreds of thousands or millions of years can be computed 

 only approximately and then the results are likely to be 

 far off. Computations have been made by geologists and 

 physicists ; the former through a study of rates of erosion 

 and deposition of rocks, the latter through a study of 

 radioactive substances. Throughout the years the esti- 

 mates of the geologists for the age of the earth have 

 varied from around 40,000,000 years to the present reck- 

 oning of at least 500,000,000 years since the beginning 

 of the Precambrian. The most recent reckoning of the 

 physicists gives nearly a billion and a half years from 

 Precambrian times, and this is probably a more nearly 

 correct estimate. The geologist is more concerned with 

 the relative and geologic ages of the rocks than the actual 

 age, and for the paleontologist the outstanding fact is 

 that there has been unlimited time for the development 

 of life. 



Igneous Rocks 



Igneous rocks are formed by the cooling of molten 

 rock. Molten rock that reaches the surface is known as 

 lava; when it does not reach the surface, as molten 

 magma. Rocks derived from molten material that 

 reaches the surface of the earth on land or below water 



