ROCK DECAY IN THE WESTERN DIVISION 329 



pact, fresh dacite, the tuffs are soft and talcose, resembling a very com- 

 pact, structureless clay. The principal alteration which the material 

 appears to have undergone is hydration. It can be easily cut with a 

 knife, and on exposure to the air it rapidly crumbles. This material 

 has not been placed in the class with the soft rock, although it might 

 properly be so classed. Since the classification shown on the sections 

 was made with a view to its practical application to engineering problems, 

 the upper limit of hard rock does not generally correspond with the limit 

 of rock weathering from the surface downward. The rock classed as hard 

 usually shows more or less alteration of its constituent minerals, but not 

 enough to, affect their coherence. While this incomplete weathering 

 does not materially affect the excavation of the rock, it becomes very 

 important and should be carefully considered when the rock is intended 

 for use in construction. Rock which appears to be perfectly fresh when, 

 first removed from the quarry often contains many incipient fractures, 

 and these develop rapidly on exposure. It is probable that all of the 

 tuff and a considerable proportion of the dacite in the Eastern Divide 

 cut would develop this weakness on exposure, and hence would be 

 entirely unsuited for structural purposes. 



ROCK DECA Y IN THE WESTERN DIVISION 



Turning now to the western division, the phenomena of rock decay 

 are found to be strikingly different, and, as already pointed out, this 

 probably depends largely on climatic differences which prevail on oppo- 

 site sides of the isthmus. The most striking difference is the almost 

 complete absence of red color in the surface soils. This change in color 

 coincides so exactly with the change in climatic conditions that it is 

 difficult to escape the conclusion that the change in color is due directl}' 

 to climatic causes. The prevailing color in the surface soil in the region 

 west of the lake is a bluish gray, varying to black. It is sometimes a 

 yellowish gray and very rarely red. One reason suggested for the ab- 

 sence of the complete oxidation of the surface soil and the consequent 

 red color is the greater amount of vegetable matter which becomes in- 

 corporated with the upper layers of the soil. As pointed out in the dis- 

 cussion of the climate, the surface soil is alternately baked and saturated 

 with water. The numerous cracks which form during the dry season 

 collect leaves and twigs and when the cracks are closed by the moisten, 

 ing of the soil this vegetable matter is thoroughly incorporated with the 

 clay to a very considerable depth. It may be that it is present in suffi- 

 cient quantity to combine with all the oxygen which is carried down by 

 the percolating waters and thus prevent the oxidation of the iron con- 



