PALEOBOTANY 579 
either decayed or became carbonized, leaving only the 
impression of its surface features. The well-known 
"fossil fern-leaves," found in coal mines are of this nature. 
The tissues of the plant were transformed into coal, 
leaving the impression or cast on the adjacent shale. The 
first stage in this process may often be observed in the 
autumn, when impressions of recently fallen leaves are 
made on the surface of wet mud. Obviously from such 
fossils we can learn nothing of internal structure. 
Petrifactions are formed by the gradual replacement 
of the organic tissue by mineral matter, usually carbonate 
of lime (CaCOs) or silicic acid (H 4 SiO 4 ). In this process 
the tissues become soaked with a saturated solution of 
the given mineral, which is gradually deposited from solu- 
tion, and takes the place of the original organic matter. 
By this means the most minute details of microscopic 
structure are preserved, even in some cases the nuclei 
and other cell-contents. 
505. Conditions of Fossil-formation. In order to 
understand how fossils come to be formed, we must pic- 
ture to ourselves certain geological processes now in 
operation the initial stages of rock-formation. Rocks 
are of two kinds, igneous and sedimentary. Igneous rocks 
result from the cooling of molten lava poured out on 
the surface or injected into crevices by volcanic action. 
Such rocks never contain fossils, as the intense heat 
necessary to melt the rock destroys all trace of organic 
matter. 
Sedimentary rocks are formed by the deposit under 
water of the sediment formed by weathering and erosion, 
and transported by streams. This deposit may occur 
along the flood-plains or at the mouths of streams empty- 
