CONDITION OF ORGANIC REMAINS. 



19 



Animal or vegetable substances found imbedded in rocks, are 

 more or less impregnated with mineral matter, and hence have been 

 called petrifactions. The process of petrifaction consists in the in- 

 filtration of mineral matter into the pores of bvone or vegetables. In 

 some instances, the animal or vegetable matter has been almost en- 

 tirely dissolved or removed, and the mineral matter so gradually sub- 

 stituted, as to assume the perfect form of the internal structure either 

 of the plant or animal. 



The process of petrifaction may be more rapidly effected than has 

 generally been supposed. In the year 1817, I paid a visit to the 

 celebrated Dr. Jenner, at Berkley, who informed me that he had 

 made several experiments upon recent bones, by burying them in the 

 dark mud from the lias clay : in less than twelve months, the bones 

 became black throughout, and when dry, they were harder, heavier 

 and more brittle than recent bone, and the surface was shining. The 

 specimens which he showed me, presented the same appearance as 

 the fossil bones in the lias clay. The effect was probably produced 

 so speedily by the presence of the sulphate of iron, and other saline 

 ingredients with which that stratum abounds. As this stratum is the 

 most remarkable of all the secondary series, for the large animal re- 

 mains which it contains, particularly of the saurian or lizard order, 

 and as the bones are frequently covered with crystals or incrustations 

 of pyrites, I will venture to hazard a conjecture respecting the man- 

 ner in which these crystals, or incrustations of pyrites, or sulphuret 

 of iron, are formed. The stratum before mentioned, contains much 

 sulphate of iron or green copperas in solution. I suppose that the 

 carbon in the animal matter had decomposed the sulphuric acid and 

 the oxide of iron, and that the sulphur andiron, in their nascent 

 state, had united, and formed the sulphuret of iron or pyrites. I 

 was led to this conclusion by reading an account by Mr. Pepys, of 

 some mice having by accident been immersed in a jar containing a 

 solution of sulphate of iron : how long they had lain there was un- 

 known, but the remains were partly covered with small crystals of 

 pyrites, which could have been formed only in the manner above sug- 

 gested. The stone surrounding the organic remains in the lias, I 

 have observed to be considerably harder than the- other parts of the 

 same stratum. The organic remains of zoophytes and shells in lime- 

 stone strata are also generally harder than the stone in which they are 

 imbedded ; and on this account when the stone has been exposed 

 to the atmosphere a long time, the organic remains rise above the 

 surface. 



Organic remains are generally coloured by the strata in v^^hich 

 they are imbedded ; in roe-stone, chalk, and the upper fresh-water 

 limestones, they approach to a yellowish or brownish white : in lias 

 bituminous shale, and dark limestone, they incline to black ; and the 

 shells in bituminous shale are sometimes filled with bitumen in a fluid 

 state. In the strata above chalk, the bones and shells retain their 



