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 706. -GEOLOGICAL NOTES ON CACAO SOILS. 



By E. H. Cunningham- Ciiaig, Government Geologist. 



The following notes have been written to serve as an explanation 

 of the geological maps that have been produced, and to render them 

 more valuable from an agricultural point of view, especially to those 

 interested in the cacao industry. 



In these short descriptions the word " rock " is used in the 

 strict geological sense, to include sands, clays, and alluvium. 



Soil, as has often been said, represents the excess of disintegra- 

 tion over transportation, an excess which is liable to be great under 

 tropical weathering conditions. The value of a soil for agricultural 

 purposes may be considered under two heads, chemical composition 

 and mechanical compostion. With the chemical compositions of soils 

 most favourable to the growth of cacao this report is not directly 

 concerned, but it is impossible to treat of the nature of soils formed 

 from different rocks without taking some account of the more impor- 

 tant chemical constituents of these rocks, and these will be mentioned 

 as need arises in the case of any soil, it being understood that only a 

 general, rather than a particular, importance is to be attached to any 

 generalizations on chemical evidence given without special research. 



The effects of the contour of the land, which seems to be dis- 

 regarded frequently in cacao-planting, will be noted briefly in the 

 case of each formation and type of sediment referred to. 



In Mr. J. H. Hart's book on Cacao, and in the opinion of planters 

 generally, the most favourable soil is said to be " loose clay, or clay 

 with an admixture of a fair proportion of sand and lime." 



Very many rocks in Trinidad are capable of yielding under dis- 

 integration soils coming under this somewhat comprehensive descrip- 

 tion. 



The principal rocks that have been observed as forming cacao- 

 bearing soils during the geological survey in 1904 are as follows : — 



(1.) Alluvium (fluviatile and marine) ... ... Recent. 



(2.) Clays of the Tertiary Series ... 



(3.) Marls ,, ,, ,, 



(4. ) Sandy clays and alternations of sand and clay in the 



Tertiary Series 

 (5.) Sandstones of the Tertiary Series 

 (6.) Limestones ,, ,, ,, 



(7.) Shales and limestones of the Cretaceous formation ... Mesozoie. 

 (8.) Metamorphic schists and limestone ... ... ? Palaeozoic. 



(1.) The alluvium of the rivers, large and small, is made use of 

 for planting in cacao to a great extent. It is evident that the alluvial 

 soils must vary considerably in character and composition according 

 to the rocks from the detritus of which they are formed. As a 

 general rule the soil may be said to consist of fine sandy clay, the 

 material being well mingled. The soil often appears much stiffer and 

 less pervious than it is in reality owing to the fine particles of clay 

 adhering to and concealing the coarser sand grains. 



The advantage possessed by the alluvial expanses are that the 

 ground is flat and easily worked, the material is well mingled, the 

 situation is generally well sheltered, watered and convenient for trans- 



Kainozoic. 



