LIFE : ITS MECHANICAL WORK AND EOCK CONTRIBUTIONS. 141 



pods and Bryozoans; Mollusks of all the divisions; Articulates, or Worms 

 and Arthropods, very sparingly ; and sparingly. Fishes among Vertebrates, 

 and very sparingly, other Vertebrates. 



Siliceous material: through Radiolarians among Protozoans; and exten- 

 sively, Spongiozoans having siliceous spicules or skeleton. 



Carbonaceous materials : sparingly through the decomposition of aquatic 

 species and the dissemination of organic matters in bottom muds. 



Phosjyhatic materials : chiefly through excrementitious matters ; sparingly 

 from shells of some of the lower Brachiopods, and of Pteropods ; sparingly 

 from tests of Trilobites, Crustaceans, and other Arthropods, and bones of 

 Vertebrates ; and animal tissues. For analyses see page 72. 



2. Relations of the Kinds of Life to Rock-making. 



The fitness of species for rock-making depends not only on the amount 

 and character of their stony secretions, but also on their geographical distri- 

 bution, and this on their relations, as regards growth, to temperature, light, 

 moisture, and the comjDosition and mechanical condition of the air, waters, 

 or soil inhabited ; the height over the land, the depth in the water, and all 

 conditions affecting growth and burial. 



Marine species of plants and animals are those most likely to become 

 fossils, and so to contribute to rock-formations ; and, among terrestrial spe- 

 cies, those that live in lakes or marshes, or along their shores or borders. 

 The reasons are two : (1) Because almost all fossiliferous rocks are of marine 

 origin ; and (2) because organisms buried under water, or in wet deposits, 

 are preserved from that complete decomposition to which many are liable 

 when exposed on the dry soil, and are also protected from other sources 

 of destruction. 



Over the land, the chance of burial is very small. Plants and all animal 

 matter pass off in gases, when exposed in the atmosphere or in dry earth ; 

 and bones and shells become slowly removed in solution, when buried in 

 sands through which waters may percolate. Vertebrate animals, as Fishes, 

 Reptiles, etc., which fall to pieces when the animal portion is removed, require 

 speedy burial after death, to escape destruction from this source as well as 

 from animals that would prey upon them. 



Among Insects the species that frequent marshy regions, and especially 

 those whose larves live in the water, are the most common fossils, as the 

 iSTeuropters ; while Spiders, and the Insects that live about the flowers of the 

 land, are of rare occurrence. Waders, among Birds, are more likely to 

 become buried and preserved, than those which frequent dry forests. But, 

 whatever their habits. Birds are among the rarest of fossils, because they 

 usually die on the land, are sought for as food by numberless other spe- 

 cies, and have slender hollow bones that are easily destroyed. Mastodons 

 have been mired in marshes, and thus have been preserved to the present 

 time ; while the thousands that died over the dry plains and hills have 

 left no relics. 



