538 ORVILLE A. DERBY 



where gneiss and granite appear in a vast and almost level plain 

 with only a slight covering of decomposed material. For this, how- 

 ever, data is lacking and it is impossible to say how much of the 

 characteristics of this region is to be attributed to climate alone. 

 Turning our attention now to other groups of rocks, some of 

 these are found to have resisted remarkably well the "torrents 

 of hot water falling for ages in succession upon hot stones" in 

 the phrase of Professor Agassiz. (The temperature of 140° to 

 150° F. recorded by Caldcleugh for rain water running over 

 exposed rock surfaces would seem to require confirmation.) In 

 crystalline rocks in which the inalterable quartz is replaced by 

 silicates susceptible of alteration, such as nepheline and sodalite, 

 much more rapid and profound decomposition might be expected 

 than in the granites and gneisses. On the contrary, however, 

 these, though decayed in places, are generally found in a sound 

 condition at or near the surface. With many of these rocks a 

 peculiar crust of kaolin and limonite forms on the surface of the 

 decayed portion which serves as a very efficient protection for 

 the inner portion of the masses. The more feldspathic types 

 (augite-syenite) seem, contrary to what might be expected, to 

 be more susceptible to decay than those rich in the silicates 

 above named. The character of the bisilicate element also 

 seems to be of considerable importance as in a recent excursion 

 to the peak of Itatiaia no difficulty was experienced in obtaining 

 satisfactory specimens from the exposed blocks, or bosses, of 

 nepheline-syenite, augite-syenite, phonolite and even tufas, 

 whereas a very extensive exposure of mica-syenite was searched 

 in vain for a perfectly sound sample of the rock. In this, as in 

 some other cases, it looks as if some of the elements set free by 

 the decay of the rock itself might be more potent for continued 

 decomposition than those derived from the atmosphere. The 

 general soundness, though many exceptions occur, of the por- 

 phyritic (phonolitic) types of these rocks is especially notice- 

 able as it is understood that in many temperate regions this type, 

 though presumed to be geologically newer than in Brazil, is con- 

 sidered to be particularly susceptible to alteration. 



