PALEOBOTANY 185 



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 

 (Fig. 80). 



Petrifactions are formed by the gradual replacement 

 of the organic tissue by mineral matter, usually carbonate 

 of lime (CaCOs) or silicic acid (H4Si04). 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 (Figs. 97 and 100). 



129. 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- 

 ing into inland lakes or into the ocean. In addition to 

 rock-sediment eroded from the surface of the land, streams 

 also transport quantities of plant (and animal) frag- 



