xii The Crust of the Earth 217 



agate, and chalcedony is also deposited from solution in 

 water, but in that case its form is not crystalline. The 

 way in which igneous rocks occur, whether poured out as 

 lava on the surface or forced as intrusive sheets between 

 beds of other rocks, greatly influences the part they take 

 in determining the scenery of a country. 



288. Sedimentary Rocks result from the consolidation 

 of sediment deposited in lakes or on the margin of the 

 sea. They are easily recognised by their structure, being 

 built up of worn rock-fragments of all sizes. Fine muds 

 consolidate into shales, sand into sandstone, gravel or 

 pebbles into fine or coarse conglomerates, sometimes 

 cemented together by the deposition of silica or carbonate 

 of lime. In consequence of their formation in lakes or on 

 the sea -shore sedimentary rocks show marks of bedding, 

 the layers or strata having been laid down horizontally or 

 nearly so. The beds of rock are not of uniform thickness 

 throughout, but thin away as the original sediment formed 

 a thinner layer of deposit far from the land. This class 

 also includes rocks formed by the accumulation of remains 

 of animal or plant life, such as decayed vegetation forming 

 coal, and the shells of mollusca or of foraminifera, the 

 skeletons of corals and other lime - secreting creatures 

 giving rise to chalk and limestone. 



289. Metamorphic Rocks. Changes are produced by 

 heat, pressure, and Earth movements so that it is difficult 

 in many cases to decide the origin of rocks. It is con- 

 venient to class all such doubtful cases as metamorphic or 

 changed. There is much difference of opinion amongst 

 geologists as to the exact way in which metamorphism 

 occurs, and we can only indicate here how some of the 

 changes may take place. Limestone subjected to heat under 

 pressure crystallises and forms marble ; a bed of clay under 

 similar influences is altered into slate. The temperature of 

 rocks deeply buried under a mass of newer sediment is 

 greatly raised ( 291), and, as the pressure of the upper 

 layers is extreme, changes of chemical composition and of 

 structure are necessarily produced. When great Earth 

 movements fold over and thrust forward masses of rock, the 



