540 ORVILLE A. DERBY 



are almost invariably quite soft even where they show no signs 

 of decay, and go to pieces by a kind of slacking process when 

 broken up and exposed to the air, though they may have 

 required blasting in the original opening of the cuttings. The 

 eruptive rocks vary greatly in their resistance to decay. The 

 porphyritic types, especially when they are amygdaloidal, are 

 often in a state of incipient, or complete, decomposition to a 

 depth quite as great, or perhaps even greater, than that noted 

 in the granite and gneiss regions, while the types approaching 

 diabase in character are often sound, or only broken up into 

 bowlders of decomposition, quite to the surface. As the decay 

 of these basic rocks affords the favorite coffee soil, the famous 

 terra roxa, the forested tracks where they occur have been very 

 generally cleared and planted, and the coffee orchards (where 

 the underlying rock is diabasic) seldom fail to show loose 

 stones and frequently continuous outcrops of rock at the surface. 

 Many of these orchards are on almost level tracts, so that the 

 comparative thinness of the capping of decomposed rock cannot 

 be attributed to excessive washing. In short, considering the 

 length of time that this region has stood as dry land and the 

 favorable topographical disposition for the preservation of the 

 products of decay, the average amount of rock decomposition 

 may be said to be surprisingly small rather than surprisingly 

 great, and where it is profound, the cause must be sought in the 

 susceptibility of the rocks themselves and not in climatic or 

 biologic conditions.^ Orville A. Derby. 



^ In treating of the organic agencies affecting the soil, Dr. Branner notes that the 

 action of earth worms is mucli less important in Brazil than in temperate regions. The 

 same remark has been made by Mr. H. H. Smith, a very acute and accurate geologi- 

 cal and biological observer. My own experience is so far in accord with these obser- 

 vations that only once has decided evidence of the action of earth worms been noticed- 

 This case however was so striking that the absence of other observations may per- 

 haps be attributed rather to lack of attention to the subject, or of favorable conditions 

 for observing the action of these humble manipulators of the soil, and not to their 

 absence. An old pasture that had been wet by a shower after a burning that had 

 completely bared the surface, was so thickly covered with the vermicular heaps 

 thrown up by earth worms that over a space of several acres it was almost impossible 

 to put down the hand without touching one or more of them. 



