62 LITHOLOGICAL GEOLOGY. 



tains the form and structure of the original wood, and burns with an empyreumatic 

 odor. Jet is a compact black lustrous lignite. Peat is imperfect coal, or partially 

 carbonized vegetable material, from modern swamps. 



On coals, see further, page 314; also, author's Mineralogy, pp. 753-760, on Min- 

 eral oils, pp. 723-730, on Asphalt, etc., pp. 751-753. 



Fossils. — From the above account of the composition of the hard 

 parts of organic beings, their influence on the composition of rocks is 

 readily inferred. 



But the fossils themselves seldom retain completely, even in the 

 case of such stony secretions as shells and corals, their original con- 

 stitution. There is usually a loss of the organic matter.. There is 

 often a further change of the carbonate of lime into a new molecular 

 condition, manifest in the fact that the fossil has the oblique cleavage 

 of calcite ; and in this change there is a loss of part or all of the 

 phosphate or fluorid. There is sometimes, again, a change to dolomite, 

 in which the carbonate of lime becomes a carbonate of lime and mag- 

 nesia. In other cases, of very common occurrence, all the fossils of a 

 rock, whether it be limestone or sandstone, are changed to silica 

 (quartz) by a silicifying process. Silicified trunks of trees, as well as 

 shells, occur in rocks of various geological ages. In some cases, fossils 

 have been altered to an oxyd or sulphid of iron, or to other ores. 



In many cases, the fossils are entirely dissolved out by percolating 

 waters, leaving the rock full of cavities. This happens especially in 

 sandstones, through which waters percolate easily, and not in clays, 

 which latter preserve well the fossils committed to them ; and hence 

 sands, gravel, conglomerates and quartzose sandstones contain few or- 

 ganic remains. 



3. KINDS OF EOCKS. 



General subdivisions. — Rocks are conveniently divided into frog- 

 mental and crystalline. 



1. Fragmental. — Rocks that are made up of pebbles, sand, or clay, 

 the particles of sand, and even of clay, being strictly fragments broken 

 from the rocks -of the globe, either deposited as the sediment of mov- 

 ing waters, or formed and accumulated through other means, — as 

 ordinary conglomerates, sandstones, clay-rocks, tufas, and nearly all lime- 

 stones. The larger part of the rocks here included are made of sedi- 

 mentary material, that is material deposited as sediments by marine or 

 fresh waters ; and are hence commonly called sedimentary rocks. They 

 are stratified rocks, — that is, consist of layers spread out one over 

 another. Many of them are fossiliferous rocks, or contain fossils. 



2. Crystalline. — Rocks that have a crystalline instead of a frag- 

 mental character. The grains, when large enough to be visible, are 



