THE RECORD OF THE ROCKS 



Almost everywhere there are breaks, due either to some 

 cessation of the supply of rock-material or to the washing 

 away by sudden local currents of rock already deposited. Or 

 again, though rock may have been laid down without pause, 

 the conditions may have changed, so that, for example, sand- 

 stone is succeeded by fine-grained clay or shale, and this 

 again by limestone. With the change of conditions there 

 was a change in the character of the animal and plant com- 

 munities living on the sea bottom or in the waters above. 

 Some creatures live at one depth, some at another; one kind 

 prefers a sandy bottom, another prefers a limy ooze, and so 

 on. Consequently, where a great thickness of rock consists 

 of layers differing in composition, it is not possible to trace 

 a single race of animals up through the whole sequence in 

 any limited region. The race migrated and its descendants 

 must be sought elsewhere. 



One of the best examples of a rock that was laid down 

 continuously through a long period of time and yet retained 

 its general character is the Chalk of southern England and 

 northern France. This soft limestone was laid down in a 

 relatively shallow and apparently calm sea, and it contains 

 many fossils of marine animals. Among these fossils sea- 

 urchins are very common, and a careful study has been made 

 of one particular genus of heart-shaped sea-urchin named 

 Micraster. A. W. Rowe, a physician of Margate, devoted 

 his holidays to collecting these heart-urchins from the Chalk 

 of England, foot by foot. He was able to show that what 

 appeared to be a distinct species found at the bottom of the 

 Chalk gradually changed into a different species found at 

 the top. This change is almost imperceptible, but it can be 

 traced in every part of the fossil shell, and it takes place in 

 the same way in all parts of the country. Here is an example 

 of evolution caught in the act. If we were to take a set of 



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