74 OUR PHYSICAL WORLD 



Sedimentary rocks are all deposited in layers (see Fig. 30). 

 Throw a handful of sand into a tumbler of water and allow it to 

 settle thoroughly. There will then be layers of sand in the 

 bottom of the tumbler, the heavy coarse material having gone 

 down first, the lighter, finer material following. So the debris 

 resulting from the distintegration of shells and coral skeletons 

 or of the igneous rocks worn to bits by the forces of erosion as it 

 deposited in the quiet depths of the seas was sorted and laid down 

 in layers whose constituent particles were now coarse, now fine, 

 depending on the strength of the currents that brought them to 

 the place of deposit. Sometimes these layers are thin; so they 

 may readily be seen even in a hand specimen, again they are 

 thick and are only to be noted at the quarry or rock cut. 



Now igneous and sedimentary rocks may be greatly altered 

 after their original formation by heat and pressure. When a 

 new lava stream forces its way up in the cracks of older rocks it 

 alters the rock with which it comes in contact. As old beds of rock 

 are heated and subjected to terrific strains and compression as 

 they are bent and upheaved when mountain chains are formed 

 they are much changed. This process is known as metamorphism 

 and the rocks so altered as metamorphic rocks. Thus limestone 

 changes to marble, sandstone to quartzite, shale to slate and 

 schist, bituminous coal to anthracite, while igneous rocks like 

 granite change to gneiss or schist. Gneiss contains the same 

 constituent minerals as the volcanic rock from which it is derived, 

 but the component grains are flattened and forced to lie with 

 their long axes in the same direction, thus giving to gneiss a 

 somewhat stratified appearance. Schists have the constituent 

 particles even more flattened, so they are scalelike. They are 

 often so soft they may be crumbled with the fingers. They are 

 named from the dominant mineral present, as chloritic schist, 

 micaceous schist. In slate the layers of the rock are easily 

 separable. Sometimes they are very thin, as in the familiar 

 school slates. Quartzite is very hard, like quartz. It also breaks 

 with a conchoidal fracture but shows the granular structure of 



