Geography of Ohio 371 



pressions on its surface would be filled with water. If water 

 appears before the atmosphere, we might thus account for an 

 atmosphere, because hydrogen and oxygen in the water mole- 

 cules are readily vaporized, and should, if present, give the grow- 

 ing planet a gaseous envelope in the form of vapor. 



Accepting the teachings of the Planetesimal hypothesis, it is 

 safe to conclude that the agents of weathering have been operat- 

 ing on this planet at least from the time it was much less than one- 

 half its present size. The earth was growing apace, through 

 accessions of unattached bodies from the original nebulous matter. 

 Thus during the time that the globe has been making at least 

 half its growth there has been a mingling of new material from 

 without and the weathered products of old material. But the 

 amount of the accessions has progressively decreased; since the 

 beginning of what is generally called "geologic time," the Arch- 

 eozoic era, the accretions are relatively negligible. Nevertheless 

 every meteorite that occasionally falls adds something to the 

 earth's mass. 



Weathering is the agent that rends old rocks and produces 

 new ones. Freezing effects, the expansion of higher tempera- 

 tures, and certain chemical changes cause disintegration ; stream 

 and wind erosion, and later transportation of the weathered 

 products to lower altitudes and into w^ater bodies are details in 

 the change from original to secondary rocks. 



First stage in rock-making . You have seen cemented masses 

 of clay, gravel, large and small stones, and possibly also organic 

 material, which had been bound together by a deposit of car- 

 bonates, of iron, or of silica, precipitated from ground waters 

 seeping through the masses? This is an early step in the devel- 

 opment of sedimentary rocks. In accordance with the coarse- 

 ness of the material thus cemented, the rock produced is termed 

 a conglomerate, a sandstone, or a shale. Later other sediments 

 may be deposited on top, the weight of which tends to further 

 indurate the lower sediments. Pressure also arises from the move- 

 ments of uplift and depression incident to the development of 

 land areas. These movements are sometimes extensive, folding 

 the rocks, or even stretching them throughout large areas. Such 

 mechanical action produces heat which is considered a very active 

 agent in altering rocks. Thus, an original mass of loose sand, 

 or of coarse and fine gravel, in time will become a sandstone or a 



