THE SCIENCE OF OBSERVATION. 461 



be made up of exactly the same minerals as the dense rock below, 

 but we notice that the mica and pyroxene are rusty and that the 

 feldspar is stained yellowish brown. The pyroxene in particular is 

 very much changed, and quickly crumbles away in the hand. It is 

 clear that there is every stage between the solid rock, and the inco- 

 herent powder at the surface of the ground. The joint planes cross- 

 ing the solid rock below may still be observed traversing the decayed 

 portion, and also many rounded areas of rock, which are seen to be 

 identical with the stone at the bottom of the quarry.* 



How shall the facts before us be explained? It has been shown 

 that the dense rock and the loose material are the same mineralogic- 

 ally, and grade from one into the other, and it is certainly rational to 

 suppose that the latter is merely a changed form of the first. Some 

 force must have been at work on the solid rock, destroying its coher- 

 ency and converting it into loose sand. If we inspect the powdered 

 rock, it will become apparent that this change has been brought about 

 mainly by the process of weathering: surface water, with its ever- 

 present acid impurities, has brought about the partial decay of the 

 pyroxene and mica and caused the disintegration of the upper 

 part of the rock. Water has not only attacked the rock from the 

 upper surface, but has penetrated to considerable depths along the 

 joint planes, working inward toward the center of each block until 

 the mass becomes completely disintegrated. This process explains 

 the concentric shells about cores of unaltered rock, each representing 

 original joint blocks, which are seen in the second photograph. All 

 our excursions into the field will show that this is not an isolated 

 case, for wherever a ledge is exposed to our view there will be found 

 a zone of weathered rock, varying in thickness from mere films to 

 many feet. 



By this process the greatest part of the materials constituting soils 

 is formed, and the flora and fauna of the earth are rendered possible. 

 Upon such products of decay the food supply of running water mani- 

 festly depends in a large measure, as will be pointed out on our 

 next excursion; and were the scope of this article somewhat larger, it 

 would be easy to show that the rock decay seen in our photograph 

 has taken place in a length of time measured by something like ten 

 thousand years. If all rock decayed as easily, and if the rate of de- 

 composition, as determined here, held good for great distances from 

 the surface, mountains two miles in height would become a prey to 



* This photograph represents a more detailed view of the quarry wall seen in Fig. 1. 

 The relation of the two views will be understood by observing the positions of the hammers, 

 which are in the same place in both photographs. These photographs, as well as some of 

 the others that follow, were taken by Mr. John L. Gardner, 2d, for the purpose of illustrat- 

 ing these pages. 



