476 



THE CHANGING WORLD 



GEOLOGICAL TIME-SCALE (Lull) 



subjected to erosion and stratification, or those which, even if they 

 may once have been sedimentary, have lost their stratified character, 

 due to crushing pressure or to transforming association with vulcanic 

 forces. Marble, for example, laid down originally in layers following 

 the disintegration of calcareous skeletons, or by the deposition of 

 dead shells of myriads of microscopic marine organisms, is metamor- 

 phosed sedimentary limestone, while quartzite and gneiss are rocks 

 that, by the action of heat and pressure, have been made over out of 

 sandstone, which was also once stratified. 



Sedimentary biology, or the horizontal arrangement of fossil remains 

 in sedimentary rocks, practically begins with the Paleozoic era, 

 although there are shadowy evidences, such as the graphite traces of 

 primitive seaweeds, showing that life occurred as far back as the 

 Archeozoic era. In the rocks of the Proterozoic era have also been 

 found scanty traces of calcareous algae, primitive sponges, and shells 

 of radiolarians, but most of the remains of life during this enormous 

 expanse of time have been obhterated. Only a part of the Proterozoic, 

 and some of the Paleozoic, era are represented in the famous walls of 

 the Grand Canyon of the Colorado. 



