372 Frank Carney 



conglomerate, and under further action of the same agents may 

 later be changed to rocks that are quite unlike the original. 



Metamorphisin. Geologic time is involved in these processes. 

 The time units of human beings give us but a slight conception 

 of geologic time. The process that changes rocks into quite dif- 

 ferent rocks is called metamorphism. Metamorphism, it is 

 thought, has altered all rocks appreciably, save those of the most 

 recent period of earth-history . Metamorphic agencies have been 

 so effective in changing some rocks that it is very difficult to tell 

 what may have been their original form. 



When we recall that man sees only the upper part of the earth's 

 rocks, never getting a view of more than a few miles below the 

 surface even when he combines numerous outcrops widely separ- 

 ated, we begin to comprehend the limitations in his study of 

 rocks. These limitations have led to his setting up standards 

 of earth-history, implying an equally short period during which 

 the agents that have produced secondary rocks have been oper- 

 ating. His not comprehending the extent of metamorphism has 

 also led to his interpreting the older crystalline rocks as primitive 

 or ''plutonic. " According to later views many rocks of the Arch- 

 eozoic may be altered sedimentary strata. 



The effects of metamorphism are variable. Sometimes a 

 familiar rock blends into its metamorphosed product. For exam- 

 ple, marble, which is an altered limestone, may be quarried in a 

 locality, on either side of which, within a distance of a few miles, 

 the marble gives place to limestone. Again, an outcrop of quartz- 

 ite, traced laterally, may blend into unaltered sandstone. 



That heat is an important factor in metamorphism, we are 

 certain, because near the line of contact between a dike of igneous 

 rock and the formation it cuts, the strata are changed, whereas 

 a short distance from the contact, they are unaltered. A flow of 

 lava coming in contact with a bed of bituminous coal may change 

 it to anthracite. Many other illustrations might be cited, show- 

 ing the effects of heat, pressure, water, and chemical agencies 

 in altering rock. 



Clastic and organic rocks. The sedimentary rocks formed by 

 assembhng worn out products of older rocks are called clastic. 

 The great bulk of our secondary strata belong to this division. 

 Not all the load which rivers carry from the higher areas to their 

 flood plains and into the ocean is deposited, and later turned into 



