6 HISTORICAL GEOLOGY 



rarely contain fossils, but volcanic ash deposited in water may- 

 be rich in organic remains, this being especially true of certain 

 portions of the western interior of the United States. 



2. River and lake deposits. River deposits often carry river 

 forms themselves, or land forms which fell into the stream and be- 

 came entombed in its deposits. Lakes offer very favorable con- 

 ditions for fossilization. " Surrounding trees drop their leaves, 

 flowers, and fruit upon the mud-flats, Insects fall into the quiet 

 waters, while quadrupeds are mired in mud or quick-sand and soon 

 buried out of sight. Flooded streams bring in quantities of 

 vegetable debris, together with the carcasses of land animals 

 drowned by the sudden rise of the flood " (W. B. Scott). 



3. Marine deposits. By far the largest number and variety 

 of organic remains are found in rocks of marine origin, because on 

 the sea bottom the conditions for their preservation have been 

 most favorable. The distribution of fossils in strata of marine 

 origin is, however, exceedingly irregular, ranging from those strata 

 which are almost entirely made up of fossils to others which are 

 nearly barren. Longshore deposits are usually not rich in fossils, 

 because of the grinding action of the waves, while deposits formed 

 in the quiet waters off shore often contain vast numbers of fossils. 

 Many conditions have produced great diversity in the distribution 

 of marine organisms throughout known geologic time: tempera- 

 ture, depth of water, supply of food, degree of salinity, nature of 

 the sea bottom, clearness of the water, etc. The oldest fossiliferous 

 strata seem to contain practically no land forms, probably because 

 land forms were but slightly, if at all, developed so early. In 

 marine strata of more recent date terrestrial organisms are often 

 found, especially in delta deposits, where such remains have been 

 swept into the sea at the mouths of rivers. 



Significance of Fossils. — It would be difficult to overestimate 

 the value of fossils in the study of earth history. They furnish 

 most important evidence regarding earth chronology, ancient 

 geographic and climatic conditions, as well as a basis for a proper 

 understanding of the evolution relations and distribution of modern 

 organisms. Leonardo da Vinci (1452-1519), the famous artist, 

 architect, and engineer, while engaged in canal construction in 

 northern Italy, saw many fossils embedded in the rocks. He con- 

 cluded that these organisms had actually lived in marine water 

 which once spread over the region. William Smith (1769-1839) 



