90 A. Heard — The Petrology of tlie Pennant Series. 



quartz grains. The clay was thoroughly washed and decanted. 

 Abundant chlorite similar to that of the sandstones was found, 

 white mica in basal cleavage flakes, some rounded grains of zircon, 

 and needles of rutile were relatively abundant. Neither pyrrhotite 

 nor pyrites was found. Carbonaceous matter formed a large pro- 

 portion of the material in the clay. 



The Sandstones below the JSTo. 2 Ehondda. 



The sandstone below the No. 2 Ehondda coal-seam is exactly 

 similar to that above, with the exception that fluorite, biotite, 

 and sphserosiderite have not yet been found. 



The presence of iron carbonate in the cement, a relatively large 

 proportion of chlorite and pyrrhotite, together with chert as an 

 original constituent, and large quantities of secondary silica, prove 

 that lithologically these sandstones are identical with those above 

 the No. 2 Ehondda. 



Summary. 



The occurrence of iron carbonate as a part of the cement in the 

 Pennant Sandstone is not extraordinary, when the large amount of 

 entombed organic matter is taken into consideration. The presence of 

 organic material and the products of decay, the so-called humus 

 acids, acting as reducing agents, would aid in the formation of 

 iron carbonate and prevent its oxidation. Much of this carbonate 

 probably owes its presence to a metasomatic replacement of calcium 

 carbonate. The granules of siderite may have been derived from 

 rocks similar to those of the Lower Coal Series of South Wales. The 

 alkaline carbonates which hold the humic substances in solution would 

 readily dissolve silica, thus becoming effective agents in the solution 

 and redeposition of silica in the rock. A solvent of this kind would 

 be formed by the impregnation of alkaline salts by carbonic acid.^ 

 The occurrence of so much secondary silica in the rock is probably 

 the result of a redeposition of silica from such a solution. No oxides 

 of iron have been found in any of the numerous separations, either 

 by means of heavy liquids or the electro-magnet ; their absence is 

 easily explained when we consider the strongly reducing environment 

 of the deposition and consolidation of the Pennant Sandstone. The 

 perfectly rounded globules of sphserosiderite may have been pre- 

 cipitated whilst the sandstones were being deposited, but more 

 probably derived from the denudation of fireclays similar to those of 

 the Lower Coal Series of South Wales.^ The dominant mineral 

 chlorite, existing as perfect cleavage flakes, is entirely distinct from 

 the secondary aggregates of chlorite in the rock, and was probably 

 deposited in its present form. I have examined a powdered specimen 

 of the Delabole Slates of Cornwall (Upper Devonian), and have found 



1 Clark, " Data of Geochemistry " : U.S. Geol. Surv., Bull. 616, 1916, 

 p. 483. 



- Gregory, Proc. Royal Soc. Edin., vol. xxx, 1910, p. 359. 



