DUCCAN TRAP IRON-CLAY. 203 



horizontal j6ints occurj very little weathering- has taken place, the water 

 being able to percolate freely downward instead of being lodo-ed and 

 retained for lengthened periods. 



This decomposition into argillo-ferruginous masses may, under 

 favorable circumstances, take place in any rock containing iron. 



From the total absence of any indications of sedimentary orio-in 

 observed in the " summit bed^^ of the trap series, one is compelled to sup- 

 pose that it originated by the decomposition of some trappean rock, 

 but it must have been a rock in which no silicious segregations such as 

 geodes were included, for no trace of thera has been observed to occur 

 in any part of the summit bed. Such a variety of trap is very rare, but 

 not absolutely unknown, and it is conceivable that from the fact of 

 the summit bed having concluded the series of outpourings and not 

 having been covered up by later formations, the waters which percolated 

 it did not contain silica in suitable quantity to give rise to the formation 

 of siliceous amygdaloids in the unweathered flow. 



The underlying trap into which the summit bed now-a-days is seen 

 to graduate at the principal sections, as Samangarh, Wallabgarh, &c. 

 (see page 181), is a very clay-like rock without any enclosed minerals, 

 and resembling in color and fineness of texture many of the purple, 

 brown, reddish amygdaloid beds occurring so largely elsewhere, but 

 differing in the total absence of vesicular cavities, whether empty or 

 filled up. There is no obvious reason why the uppermost flow may 

 not have been of the same homogeneous clayey character, though with 

 a larger admixture of iron in its constitution. If such a rock be 

 granted, no difficulty remains in supposing that the formation of the 

 iron clay by percolation of water of similar character had occurred 

 at different periods during the trappean era, and given rise to the forma- 

 tion of similar rocks, as seen now in the several series of lower-lying iron 

 clays (see page 209). None of these, however, approach the summit 

 bed flow in magnitude or extent. 



( 203 ) 



