98 collector's manual 



and mineral-bearing strata. The United States National Museum 

 in Washington maintains a large collection of such reference fossils 

 to help geologists identify their fossil specimens and thus classify 

 and map rock formations. 



Speaking in general terms, fossils are found in sedimentary 

 rocks, that is, rocks that have been laid down in bodies of water 

 such as seas or lakes. Flood deposits of rivers and wind-laid 

 deposits such as sand dunes often contain invertebrate fossils or 

 plants although the specimens are likely not to be well preserved. 

 Searching for fossils in igneous rocks — those formed from molten 

 magmas or lavas — obviously would be a waste of time. The 

 same is generally true of metamorphic rocks which are composed 

 of sediments or other materials more or less profoundly changed 

 during times of mountain-building. 



Limestone, sandstone, and shale constitute by far the greatest 

 percentage of sedimentary rocks. Generally, ancient sediments 

 can be recognized by their arrangement in layers or strata of 

 different kinds of rock of varying colors. These layers are often 

 flat as originally laid down, but in mountain-building they may 

 be much folded so that their edges protrude from the surface at 

 various angles. No specific rules can be given as to what kind 

 of rock will contain the most fossils. In general the finer-grained 

 rocks, such as the limestones and calcareous shales, contain more 

 and better-preserved fossils than the coarser sandstones. Never- 

 theless some coarse sands contain numerous fine fossils. The best 

 rule is to examine any exposure of sediments for fossils. 



Fossils are preserved in many ways, varying from the entire 

 animal to traces such as a footprint or burrow. Most fossils 

 consist of the hard parts or skeletal portions only, preserved 



