540 Malcolm Madaren — The Origin of certain Later ites. 



superficial deposits of Imlia, though, in the form of lithomarge or 

 Lalloysite, it occurs in abundance at shallow depths.^ 



The equation most fitted to Indian conditions is, however, one 

 more in accordance with the general conclusions of Kahlenberg and 

 Lincoln, and one also indicative of more energetic chemical action in 

 the presence of abundant carbon dioxide. Here the silicates are 

 completely decomposed, all the silica passing off as colloidal silicic 

 acid. The action may, and doubtless often does, take place through 

 an intermediate sericite or lithomarge stage, but the simplest ex- 

 pression is adopted. 

 2NaAlSi3 0g + 15H30 + CO3 = Al3(0 H)e + NaoCOg + GH^SiO^. 



By this process of carbonatiou aluminium hydrate may similarly 

 be derived from any other of the felspar famil}', from the micas, from 

 the felspathoids, from the scapolites, and from many of the minerals 

 of contact metamorphism. We have, therefore, no difficulty in finding 

 an origin for all the aluminous hydrates of laterites. By carbonation 

 also ferrous and probably manganese carbonates in like manner result 

 from the decomposition of the ferro-magnesian silicates. It is, of 

 course, not insisted that any of the resulting compounds are definitely 

 formed, but rather that they are merely potential salts in ionized 

 solutions. 



So far the equations given are in accordance with numerous data 

 and with generally accepted principles. Tlie origin of the ilmeuite 

 and titanic acid met with in most laterites is by no means so oJear, 

 but it may at once be conceded that their manufacture is certainly 

 a task from which even the stoutest-iiearted of bacteria might recoil 

 without incurring the odium of pusillanimity. Data concerning the 

 alterations of titaniferous minerals are scanty. Kutile and ilmeuite 

 may, as suggested by Van Hise,- be derived from original sphene : 



(1) Ca Ti Si O5 + C Oo = Ca C O3 + Ti Oo + Si 0. ; 



(2) Ti Oo + Fe C O3 ^ Fo Ti O3 + C Oo ": 



the last equation being reversible. A careful examination of thin 

 sections failed to reveal any leucoxene in the laterite, but this form 

 of titanic acid, unless under exceptional conditions, would be dis- 

 tinguishable with great difficulty from the bauxite matrix. 



Titanic acid is, however, by no means confined to laterites and 

 bauxites among superficial deposits. The extensive soil analyses of 

 Dunningtou^ showed its presence in all the 72 soils gathered from 

 many parts of the world and examined by him. The average per- 

 centage of Ti Oo obtained for the whole series was '95. When to 

 this normal percentage is added that collected by the exceptional 

 facilities enjoyed by laterites for selective segregation, the high 

 percentage contained by many bauxites is perhaps no matter for 

 surprise. 



We have thus derived by the agency of carbonating waters from 



^ [A more satisfactorj- formula to represent the process of sericitization in alkali 

 felspars is that given by Van Hise (Metamorphism, p. 254) : 3 K Al Sij 0^ -f- II" -f- 

 C Oo K H., AI3 Sia 0,, + 6 Si Oo + K., C Oa.— Edit. Geoi.. Mag.] 



2 U.S. Geol. Surv., Mon. 47 (1904), p. 227. 



' Amer. Joura. Sci., vol. xlii (1891), p. 491. 



