Abstract

The weathering of volcanic rocks in a humid area results in a soil of halloysite, gibbsite, and quartz, whereas in more arid areas weathering produces montmorillonite. The rivers of Puerto Rico reflect the climatic conditions in their watersheds insofar as the characteristic top-soil components in their load is concerned. Because of the small volume of clay relative to the larger volume of mineralogically unstable volcanic sand in the rivers, however, the bulk sediment being deposited off-shore provides only slight indication of climatic conditions on-shore. This observation indicates that ancient source area climatic conditions inferred from clay fractions of sedimentary rocks are always tenuous, particularly in view of the fact that some (if not all) of the clay fraction of a rock may be derived from altered unstable sand grains.

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