136 DYNAMICAL GEOLOGY. 



indurated. The St. Peter's sandstone afforded similar facts. In one case the cavities 

 over the exposed surface had a lining of quartz crystals, while the rock a few inches 

 below had the common friable character. The effects were connected in some way with 

 weathering processes. In some cases of the kind the silica may have come from the 

 decomposing action of percolating acid waters on feldspar grains sparsely disseminated 

 through the rock. 



Over the cold bottom of the ocean some silicates have been formed. Among them 

 are masses or concretions of bronzite, a silicate of magnesia and iron related to pyroxene, 

 and small crystalline groups of Phillipsite (Christianite). At depths of 2200 fathoms and 

 over, the pressure on the bottom is 5000 to 12,000 pounds to the square inch ; and this 

 may favor the production of silicates, where the siliceous parts of Sponges, Diatoms, or 

 Radiolarians abound, with the results of the decomposition of volcanic dust and pumice. 

 Another silicate of common occurrence, forming in shallow water as well as in deep, is 

 the green-sand called glauconite, a hydrous silicate of iron and potash. 



Chemical Work of Living Organisms. 



Respiration in animals, and also in plants, is a means of introducing oxy- 

 gen from the air to carry on processes of oxidation among the elements in 

 the structure, and the excretion of carbonic acid is one prime result. The 

 growth of green plants, however, depends on a deoxidation process, the car- 

 bonic acid of the air being decomposed in the sunlight by the green color- 

 ing-matter (chlorophyll) of the plant, its carbon forming the food of the 

 plant and its oxygen being set free. Plants of the Fungus division (Mush- 

 rooms and the Microbes) are not green (have no chlorophyll), and cannot 

 get their food directly from the carbonic acid in the air. The chemical work 

 of life of most geological importance, apart from the making of coal and 

 related products, is that carried on by the lower plants ; and only this is here 

 briefly considered. 



Plants, and especially the lower Cryptogams, contribute chemically to 

 geological change through their roots or the libers with which they come 

 -in contact with rocks. The acidity of roots is often very decided, as is 

 manifest from the furrows they make in the surfaces of stones, and especially 

 in limestones. Roots of plants germinated in sand over a slab of marble 

 leave an imprint on the marble. Professor Storer observes that "it is to be 

 noted that this action by chemical corrosion through the roots is incessant 

 and continuous." The lichen Stereocaulon Vesuviai^um, which grows on 

 rocks, and among them on Vesuvian lavas, affords one ninth its weight of 

 ash ; which from one Vesuvian specimen, according to Roth, contained silica 

 46-41, alumina 19-67, PeA 6-88, FeO 4-17, magnesia 5-23, lime 10-53, soda 

 2-02, potash 4-09 = 99-00. For other analyses, see page 75. 



The microbes, or Bacteria, are at the bottom in much of the world's chem- 

 istry. They do not get food from carbon dioxide, but, like true Fungi, find 

 it in other compounds : for example, those consisting of carbon, hydrogen, 

 and oxygen, as sugar, starch ; or those containing these elements and nitro- 

 gen, etc., as albumen, muscle, or even a mineral sulphate ; they taking 

 the part of the compound required for food, and leaving the rest to 



