STRATIFIED ROCKS. 201 



preserved in ice. In the frozen cliffs and soils of Siberia, 

 the carcasses of extinct elephants and rhinoceroses have 

 been exhumed by the rivers, in a condition so perfect that 

 dogs and wolves fed on the flesh. In peat-bogs are found 

 the perfect skeletons (still retaining the organic matter of 

 the bones) of extinct animals ; and in some cases even the 

 flesh is preserved, but changed into a fatty substance 

 (adipocere). These are all in comparatively recent strata. 

 But, even in the oldest strata, organic matters of once 

 living beings are preserved, though changed into coal, 

 lignite, petroleum, bitumen, etc. 



2. Organic Structure preserved. This is the type 

 of what is called petrifaction ; it is best illustrated by 

 petrified wood. In many strata, but especially in the 

 sub-lava gravels of California (page 395) and the tufa 

 beds of California and the Basin region, drift-wood is 

 found completely changed into stone. In these we have 

 not only the form, not only the general structure i. e., 

 bark, wood, and pith, concentric rings, medullary rays, 

 and woody wedges but even the minutest microscopic 

 structure of tissue and markings on the walls of cells, 

 perfectly preserved in the stony matter (usually silica) 

 replacing the wood. 



Mode of Petrifaction. It must not be imagined that 

 the wood is turned to stone, but is only replaced by stony 

 matter. As each particle of woody matter passes away 

 by decay, a particle of mineral matter is deposited in its 

 place from solution, thus reproducing its structure per- 

 fectly. Wood best illustrates the process, but in a simi- 

 lar manner the minute structure of bones, teeth, corals, 

 shells, etc., are preserved, even though the original mat- 

 ter is all gone. The most common petrifiers are silica 

 and carbonate of lime. 



3. Organic Form only preserved. In many cases 

 the structure is not preserved, but we find only a mold of 

 the external form, or a cast of the same in stone. This is 



