GEOLOGY OF THE PHILIPPINE ISLANDS. 337 



the Philippines has been derived from the disintegration of rocks poor 

 in quartz. It is more in the nature of an arkose, and hence yields a better 

 soil than is usual for sandstone. The shales of the coal measures, next 

 to the volcanic rocks, I believe, yield what may be classed as among the 

 best residual soils. 



In temperate regions we must consider several varieties not found in 

 the Tropics; for instance, glacial soils and loess or wind-borne soils, but 

 in the present discussion concerning the soils of the Philippines only the 

 water-borne material, fluvial, and subaerial soils remain for consideration. 



Examples of fluvial soils are seen in the valley of the Cagayan in 

 northern Luzon, or in the Cotabato Valley, Mindanao. These soils are 

 very rich, not only because of their chemical heterogeneity, but also 

 because of the mechanical state of their particles, they being in an un- 

 usually finely divided condition and hence more easily utilized by plants. 

 Furthermore, in a broad meandering stream the soil is being continually 

 worked over by the lateral cutting and filling familiar to one who has 

 much acquaintance with rivers. 



The subaerial deposits are water-borne,, with the difference that the 

 material is not carried along definite channels, but is spread out sheet- 

 like in fans and cones at the bases of the mountains. The material is 

 always much coarser than that just refered to and therefore not so good. 



A heavy, sticky, grayish-blue soil, which corresponds pretty closely to 

 the "adobe" in California, occurs at the bases of many mountains in the 

 Islands. This is derived largely from the decomposition of basalts and 

 similar rocks. This soil, because of its impervious character, is especially 

 good for rice growing, as it holds water for a long time. 



From my own observations, while in the field working on other matters 

 more strictly geologic, I have noted that the relation between the agri- 

 cultural products and the geologic formations in general is shown in the 

 following table : 



Formation. 

 (Laterite Sugar, coffee, camotesr 7 some hemp, coco- 

 Residual I 



Limestone — _ Corn, camotes. 



Sandstone and shales.... Hemp. 



Transported.... | Fluvial TobaCCO - 



(Subaerial Hemp and some sugar. 



As I have made no special studies along this line, this list should be 

 regarded as only a rough tentative summary. It is hoped that it will be 

 suggestive to scientific agriculturists. 



Artesian water. — The principles governing the occurrence of artesian 

 water are so generally known that they need not be repeated. Still, it is 



27 Sweet potatoes. 



