GEOLOGY 



GEOLOGY 



They are quartz, feldspar, mica, hornblende, 

 augite, clay, carbonate of lime and talc. (Most 

 of these are described in these volumes. See 

 also MINERALS AND MINERALOGY.) 



What the Rocks Tell. As the next step in 

 their lesson, Tom and his classmates learned 

 in what way the rocks bedded in the earth 

 throw light upon its history. Could the sur- 

 face layer of mantle rock be removed from 

 the land, that is, the upper layer of soil, clay, 

 sand, gravel and broken rock, there would be 

 exposed a zone of solid rock, possibly many 

 miles thick. About four-fifths of this consists 

 of rock arranged in layers, that is, stratified 

 rock; the other fifth is crystalline. The crys- 

 talline rocks are of two types, igneous and 

 metamorphic. This latter term is applied to 

 rocks which exhibit marked changes from their 

 original condition. Therefore metamorphic 

 rocks may have been either stratified or 

 igneous in their first state. 



An examination of the successive layers of 

 stratified rock shows that sometimes the dif- 

 ferent beds of rock have been deposited in 

 regular order, one on top of another, with- 

 out any alteration in the position of the sur- 

 face on which they were deposited, and in 

 other cases the underlying layers have an 

 angle of inclination different from that of the 

 overbying beds. But whatever their position, 

 we know that the overlying beds must have 

 been deposited at a later period than the deeper 

 ones, and so through these various layers we 

 read backward to learn about the earth. The 

 fossils in the sedimentary rocks indicate some- 

 thing of the kinds of plants and animals that 

 lived when the rocks in which they are found 

 were formed, and the pupils were intensely 

 interested when their teacher told them that 

 the youngest beds of rock prove that the life 

 which existed when they were deposited bore 

 marked resemblances to life of the present 

 time. 



Fossils in deeper rocks show more and more 

 variations from life of to-day, and when the 

 lowest that contain distinct evidences of life 

 are reached, the evidences point to types far 



removed from those that we now know. The 

 pupils were curious to learn if rocks had been 

 found giving no evidences whatever of life 

 on the earth when they were deposited. Their 

 teacher told them that so far as geologists 

 could learn, life may have existed when the 

 most ancient rocks were deposited, for rock 

 formations apparently without any fossils con- 

 tain graphite beds or carbon-bearing slates 

 and limestones. Carbon is a characteristic 

 element of both plant and animal organisms 

 and graphite is nearly all pure carbon. Fur- 

 thermore, lime carbonate, of which limestones 

 are composed, is formed through the agency 

 of plants and animals. 



The order of the formation of igneous rocks 

 to a certain extent is revealed -by the relation 

 they bear to the stratified. In some cases the 

 latter are broken through by lavas, showing 

 that the stratified rocks were the earlier 

 formed. In the majority of cases the meta- 

 morphic rocks lie beneath the sedimentary lay- 

 ers and are often broken through by igneous 

 rocks. In the lowest rock series there is much 

 igneous rock, and geologists generally agree 

 that there was an early period of dominantly 

 igneous activity. It is probable that this re- 

 mote igneous era involved rather widespread 

 volcanic conditions similar to those which oc- 

 cur locally at the present time. Throughout 

 all geologic history sedimentary and igneous 

 rocks have developed contemporaneously. 



The story told by the rocks is not an un- 

 broken record by any means, for no one has 

 ever been able to proceed directly downward 

 through the entire series of bedded rocks. 

 Geologists have made their investigations from 

 edges of various rock layers in places where 

 by warpings or tiltings they have been brought 

 to the surface, or where they have been ex- 

 posed by the wearing away of overlying beds. 

 Here and there, in one land and then another, 

 throughout the world, the story has been read, 

 and the records have been pieced together as 

 completely as is possible with the data avail- 

 able. To the average person the amount of 

 information gathered seems incredible. E.S. 



The Story of the Earth 



Its Origin. Tom and his classmates were 

 now ready to follow their teacher far afield 

 to learn the story of the earth. Question after 

 question came to their minds. How was the 

 earth formed? How did it reach its present 

 condition? How long did it take for these 



changes to occur? Could such questions be 

 answered? If so, how? -These and many sim- 

 ilar questions the geologists have attempted 

 to answer, and some of their conclusions rest 

 on a reasonable basis of fact. Others they 

 have to admit are unanswerable. 



