GEOLOGY OF THE AREA. 1 7 



k£d. In the side of the cliffs of this ridge a sheet of felsite of con- 

 siderable thickness is seen to be intruded into the gneiss. The dip 

 of the sheet is low and to the west. 



Laterite, 



There are two chief varieties of laterite found in South Malabar, 

 which may be named the vesicular and the pellety 1 varieties. 



The vesicular laterite is a ferruginous hardened clay, penetrated 

 by numerous vermicular, branching, and anastomosing tubes, which are 

 usually about half an inch or less in diameter. In the parts that have 

 not been exposed to the air, these tubes are filled with a white or 

 yellow clay containing a much smaller percentage of iron than the 

 walls of the tubes, and a much larger percentage of potash. The 

 deeper we dig into the laterite and the less affected it is by the 

 weather, the fainter becomes the distinction between the walls of the 

 tubes and their contents, till at last it may disappear altogether, and 

 what is laterite above is simply a clay below. This shows, if it were 

 necessary, that the peculiar structure of laterite is due to a kind of 

 concretionary action — the iron tending to segregate in the form of 

 tubes, from which, when exposed, the clayey, non-ferruginous parts 

 are washed out. The exposed laterite has therefore a higher per- 

 centage of iron than the clay from which it was formed. The pas- 

 sage downward from laterite with much iron to an ordinary clay is 

 seen in the railway cutting a short distance south of Tirur Station. 

 The surface of the hill through which this cutting is made is of 

 typical laterite, while the deeper parts of the section show only a 

 somewhat sandy, irregularly laminated clay. 



This form of laterite is easily dissolved by water wherever there 

 is a surface of contact between the water and the air. When the 

 side of a well or pond is of laterite, irregular little caverns are 

 hollowed out at the surface of the water. Usually long vertical 

 tubes of considerable width are formed at the same time. They are 



^his is generally distinguished from the "pisolitic laterite" of the east coast by 

 the much smaller amount of cementing material present. The vesicular laterite is nearly 

 the same on both coasts. 



C ( 217 ) 



