258 Robert Harkness, Esq., on Fossil Footprints. 



of worm casts, and the occasional occurrence of tracks 

 caused by wandering molluscs, point out the former ex- 

 istence of animals which have left no trace of the organised 

 forms ; and but for these results of their mode of life we 

 should have been in ignorance of their having been occu- 

 pants of the shore in the era of the Variegated sandstone. 

 The occurrence of ichnolites is abundant evidence of the 

 former existence of the animals which caused them, and 

 it by no means follows that because we have not suffi- 

 cient, either in the form of organic remains or in tracks, or 

 other evidence that no other animals co-existed with the 

 Cheloniae, the Sauriee, and the Batraehise ; and this may arise 

 from the circumstance, that these animals were incapable of 

 leaving such evidence, for it is only from animals furnished 

 with feet that we can expect footmarks, or from worms or 

 wandering molluscs that we should have tracks. The absence 

 of organic fragments from the Variegated sandstone is in all 

 probability owing to this deposit being of such a nature as to 

 be hostile to their preservation. The large quantities of 

 peroxide of iron which abounds in the sandstone of its bed 

 must have exerted an injurious influence on any organic 

 bodies which may have been contained therein. Animal or 

 vegetable matter, subjected to the agency of this substance, 

 is rapidly decomposed, the peroxide of iron furnishing an 

 equivalent of oxygen to aid in the decomposition of car- 

 bonaceous matter, which, being converted into carbonic acid, 

 unites with the protoxide of iron, forming carbonate of iron, 

 which, on exposure to the atmosphere, is rapidly converted 

 again into peroxide, and is in this state again in a position 

 to effect a further change on other organic substances. By 

 this means the carbonaceous portions were doubtless re- 

 moved from the bones which may have been buried in the 

 sand, and the percolation of water charged with carbonic 

 acid, would speedily remove all traces of calcareous matter. 

 It is probable that to these circumstances we owe the ab- 

 sence of organic remains in the Variegated sandstone. And 

 the same circumstances, where they have occurred, would 

 also deprive other formations of their organic contents. 

 Sandstones, generally, are deficient in organic remains, and 



