lO AN INTRODUCTION TO THE STUDY OF FOSSILS 



preserved that dogs and wolves will eat their flesh. Likewise, 

 remains of the cliff dwellers in Arizona and New Mexico, such 

 as clothing, food, and human bodies, have been preserved in 

 the dry air of that plateau region for at least many hundreds of 

 years. Many Tertiary insects have been preserved in their 

 entirety, except for complete desiccation, without a trace of 

 mineral infiltration, inclosed in amber ; upon this ancient gum 

 as it was exuding from the pine trees, insects settled, and being 

 held fast by the sticky substance, were finally entirely inclosed. 

 Such trees and their gum wxre buried in the sediment beneath 

 the waters of seas or large lakes, thus preserving these ancient 

 insects in all their perfection of form and brilliant coloring. 



Usually, however, only the hard parts of organisms are pre- 

 served, and but few of these are found in an unaltered state in 

 pre-Cretaceous rocks ; it is principally in the Cenozoic rocks 

 that they thus occur. The unaltered bone is more or less 

 spongy in appearance, something like volcanic pumice or slag, 

 but most of the cavities are canals, continuous and anastomos- 

 ing ; they do not end blindly as do those in slag. These Haver- 

 sian canals, filled during the life of the animal with arteries, 

 veins and nerves, are much larger and more numerous in the 

 interior of the bone than in the more compact and consequently 

 harder outer portion. Non-petrified shells of Cenozoic or older 

 age, besides having lost much or all of their original color, are 

 usually also somewhat chalky in appearance. This chalkiness 

 is due to the loss of the fleshy material which penetrates the 

 living shell, not only vertically, but also horizontally. The 

 abundance of this spongy animal matter may be demonstrated 

 by an experiment. If a recent shell be placed in a dilute acid 

 which is changed frequently, in two or three days the calcium 

 carbonate will be dissolved away but the shape of the shell will 

 still be retained by the animal matter which formerly penetrated 

 it. Thus shells from the Sankaty beds (Pleistocene) of Nan- 

 tucket, Massachusetts, differ from the recent shells only in their 

 porosity and in having lost much of their color. 



