Vol. 54.] TRACHYTE FJROM CLTPPERTON ATOLL. 231 



The secondary substance, when examined with a high power, often 

 shows concentric or agate-like modes of aggregation ; and, as the 

 silicification of acid and intermediate igneous rocks is by no means 

 an uncommon phenomenon, it was thought at first that the white 

 rocks were merely silicified trachytes. A chemical examination at 

 once disposed of this idea. The most altered rock possesses the 

 following percentage composition : — 



SiO., (2-8) and matter insol. in HCl 5-0 



P2O; 38-5 



AI.O3 25-9 



PeA 7-4 



Loss on ignition 23*0 



99-8 



Thus 95 per cent, of the rock is a hydrated phosphate of alumina 

 and iron, somewhat allied to the so-called redonite from the island of 

 Eedonda, in the West Indies.^ * 



The appearance of a microscopic section of this rock is represented 

 in PI. XXIII, fig 2. The outlines of one of the porphyritic felspars 

 are clearly seen, but the original substance has been replaced by the 

 phosphate, which shows the peculiar agate-like structure above 

 referred to. The groundmass has also been replaced by the same 

 material. 



To complete the chain of evidence, an intermediate specimen, in 

 which the phenocrysts were comparatively unaltered and the 

 original structure of the groundmass was well preserved, was 

 partially analysed. It contained 43*74 per cent, of silica, 17 per 

 cent, of phosphoric anhydride, and gave 12'3 per cent, loss on 

 ignition. 



The three specimens above described illustrate very clearly the 

 progressive substitution of phosphoric for silicic acid, and the intro- 

 duction of water : — 



I. II. III. 



SiO^ 54-0 43-7 2-8 



PA 8-4 17-0 38-5 



Loss on ignition...... 3'8 12*3 23-0 



A comparison of the whole series of microscopic sections proves 

 that the phosphatizing process first attacks the interstitial matter, 

 then the microlitic felspars of the groundmass, and last of all the 

 porphyritic sanidines. 



The chemical analyses show that the change is accompanied by 

 the removal of silica and alkalies, and by the introduction of phos- 

 phoric acid and water. This change has probably been effected by 

 solutions of alkaline phosphates, principally ammonium phosphate, 

 and other compounds derived from the droppings of sea-birds.^ 



. 1 C. U. Shepard, Am. Journ. Sci. vol. xlvii (1869) p. 428, and vol. 1 (1870) 

 p. 96. 



^ Some additional specimens from Clipperton have been received from 

 Admiral Wharton. They are portions of coral-rock more or less phosphatized. 



