264 The National Geographic Magazine 



globe. What have been some of the 

 more active forces which have broken 

 up this congealed mineral matter and 

 brought it into the present condition in 

 which we see the surface of our globe ? 

 First of all I will speak of the action of 

 water, which is and has been one of the 

 chief disintegrating agents acting upon 

 the earth's surface. At the time the 

 first crust was formed over the surface 

 of the earth all the water which now 

 exists must evidently have been above 

 the earth's surface in the form of steam. 

 As the cooling progressed this steam 

 tended to condense in the form of clouds 

 and finally water. Thus the original 

 rain falling upon the hot surface of the 

 earth was at once converted again into 

 steam, but not until it had started a cer- 

 tain solvent action. Water has been 

 termed the universal solvent, and it is 

 not difficult to see how active it must 

 have been at the time of which I speak. 

 The sudden cooling of the surface at the 

 spot where a drop of water struck would 

 tend to crack it, the hot water would 

 dissolve quickly any of the substances 

 soluble therein, and this continual bom- 

 bardment of boiling water must have 

 had a tremendous effect in disintegrating 

 the original crust formed over the earth's 

 surface. As the earth continued to cool 

 and diminish in size, the original surface 

 wrinkled and formed hills and valleys. 

 The continual descent of water would 

 filial^ permit some of it to remain in the 

 liquid state upon the earth's surface, 

 and this coursing down the valleys con- 

 tinued the disintegration, both by solu- 

 tion and attrition. The original mineral 

 matters were thus brought into a form 

 of solution or suspension, and, seeking 

 their natural chemical affinities, began 

 to form from the first igneous rocks the 

 first sedimentary rocks. These are the 

 rocks which we now see in strata, cover- 

 ing the greater part of the earth's sur- 

 face. All these stratified rocks must 

 have been laid down under the water, 

 and thus we are convinced that the sur- 



face of the earth during the long period 

 of the formation of the soil must have 

 been alternately above and below the 

 surface of the water collected upon the 

 globe. 



INFLUENCE OF ORGANIC LIFE 



When organic life came upon the 

 earth's surface a new disintegrating 

 force was introduced. Organic life, even 

 in its smallest forms, such as bacteria, 

 acts with vigor in decomposing rocks. 

 The larger forms, which produce root- 

 lets, help this disintegrating process 

 along. These roots find their way into 

 crevices of the rocks, and tend to split 

 them open and to admit water below 

 their surface. Certain bacteria also 

 tend to oxidize the nitrogen of the air 

 and form nitric acid, known under the 

 common name of aquafortis, which has 

 a vigorous solvent action on many kinds 

 of rock. 



DIFFERENT KINDS OF SOIL 



111 the process of further cooling, ice 

 was formed, and this also tended to 

 have a disintegrating influence. Water 

 in passing into ice increases in volume, 

 and this tends. to break and disintegrate 

 many bodies. Rock saturated with water 

 thus tends to break up when the water 

 becomes ice. During the perio 1 of the 

 ice age when large glaciers moved over 

 the earth's surface, the crushing and 

 grinding effects of the ice had much to 

 do with disintegrating the rock. The 

 vast areas of glacial drift which form 

 the soil of many of our Western States 

 are evidences of the gigantic scale on 

 which these ice mills of the gods slowly 

 ground the stones of the earth into soil. 

 When the soil is formed by the decay 

 of rocks without the transporting action 

 of water or ice being active, the soils 

 are said to be formed in situ. When 

 the products of soil disintegration are 

 carried by water and deposited along 

 the banks of the streams or at their 

 mouths, the soil is called alluvial. When 



