398 



GENERAL BIOLOGY 



coverings of the land's surface took place at much later periods in some 

 places than in others. 



To return to the fossils themselves, it is necessary for the student 

 to understand the various forms in which fossil remains come down to 

 us. Bones may be buried in silt which then hardens. Later, water, con- 

 taining" minerals, may make its way through the silt and bit by bit dis- 

 solve the bone, and deposit a mineral in its place. This is petrification. 

 The shape and form of the bone remain intact, though the original bone- 

 substance is replaced by a mineral. 



Fig. 246. 



Mammoth found frozen in Siberia. The skin is mounted 

 in the museum of Petrograd in the posture in which, it was 

 found. (From Lull's 'Organic Evolution,' by permission of 

 the Macmillan Co., Publishers.) 



Or, an organism may retain its form long enough to have the sur- 

 rounding substance completely encase it and harden. As time goes by 

 the organism is dissolved and disappears, while the hollow space it occu- 

 pied remains. Such hollow forms, in the shape of organisms no longer 

 present, are called molds. Investigators fill these molds with a material 

 such as plaster of paris (which hardens easily) and obtain a cast of the 

 original organism. 



Then, too, as stated above, the tremendous pressure of the upper 

 layers may crush the fossil forms beneath, or the minerals which caused 

 petrification may be re-dissolved, so as to expose the fossil remains to 

 conditions which destroy them, and this may happen after they have 

 been so encased for thousands of years. Weathering and erosion may 

 also expose fossils to the harmful effects of the weather. 



When very definite fossil remains are always found in certain strata, 

 they are often called "index fossils," as such fossils can be used to de- 

 termine the place and period of the strata from which they are taken. 



One of the most interesting finds in 1901 was that of an ancient 



