ELECTRON-MICROSCOPY OF FOSSILS — HAY 415 



continents, but are present in a few areas. Much of the work of the 

 geologist consists of correlating strata from one place to another, 

 either by studying the nature and sequence of the rocks or by using the 

 fossils found in the strata. A geologist who specializes in correlating 

 strata is called a stratigrapher ; a paleontologist who specializes in 

 using fossils to establish the age equivalence of strata is called a strati- 

 graphic paleontologist. Stratigraphers and stratigraphic paleontolo- 

 gists have had great success in unraveling the geologic history of large 

 areas, but a very troublesome problem has always remained — how to 

 correlate across the ocean basins. The fossils used in correlating beds 

 in the shallow-water deposits which cover much of the continents often 

 cannot be used to correlate across vast distances. At the present time, 

 faunas in widely separated geographic areas are often strikingly 

 different, and the same is true of the past. Faunal provinces have 

 existed through all of geologic history, and they have presented a 

 barrier to intercontinental correlation and detailed examination of the 

 earth's history. 



The remains of planktonic organisms of the oceans are found only 

 rarely on the continents, but where they are present they offer an 

 unparalleled opportunity for intercontinental correction. The plank- 

 tonic Foraminifera have been used for this purpose smce the 1930's. 

 They have permitted detailed correlations between North and South 

 America and the Mediterranean region. However, it is becoming 

 apparent that certain planktonic Foraminifera also are restricted to 

 particular faunal provinces, and it is excedingly difficult to correlate 

 deposits in northern Europe, for example, with those of the Mediter- 

 ranean region. Coccoliths settle much more slowly than the tests of 

 planktonic foraminifera and the boundaries of coccolithophore prov- 

 inces (if they exist) are obscured. At present it seems as though 

 coccoliths may be ubiquitous in their distribution. They have a fur- 

 ther advantage for purposes of correlation — they show extremely 

 rapid evolution and a number of radical changes have taken place, 

 making different species easy to recognize. 



Little research has been carried out on the detailed ultrastructure 

 of coccoliths, but high resolution electronmicrographs show fine struc- 

 ture, in the form of markings spaced about 100 A apart on the surface 

 of the elements which make up the coccolith (pi. 4, fig. 2) . The nature 

 of these markings is still a matter of conjecture, but they probably 

 reflect regular changes in the structure of the crystals of which the 

 skeletal elements are made. Further investigation of skeletal ultra- 

 structure at high magnifications is sure to produce interesting results. 



From these examples it can be seen that the electron microscope is 

 a tool that will play an important role in paleontologic research m the 

 future. 



