FIRST FORMS OF LIFE. 27 



LOWER AND UPPER SILURIAN FORMATIONS 



FIRST FORMS OF LIFE. 



It follows that the strata in their order might be described 

 as a record of the state of life upon our planet from an early 

 to a comparatively recent period. It is truly such a record, 

 but not one perfectly complete. For neither are we to sup- 

 pose that every kind of organism has been entombed in the 

 matter afterwards hardened into rocks, nor are we to pre- 

 sume that examples of all which have been so entombed are 

 now liable to detection. Those composed of soft substance 

 only, even if so entombed, would have little chance of leaving 

 any trace of their form and character behind them. Some 

 rocks have been subjected to so great a change by means of 

 heat, that any organisms involved in them would be almost 

 sure to disappear. There is, moreover, but a limited portion 

 of the earth namely, that uncovered by sea open to in- 

 spection, and of this but a few scattered parts have been in- 

 spected : we cannot tell what blanks there may be in the 

 Stone Book, yet to be filled up by more extensive and happier 

 inquiry. It is therefore necessary, in looking over this sin- 

 gular chronicle, to make certain allowances for falterings and 

 shortcomings, even while we pause breathless in admiration at 

 the wondrous historical detail which it has so unexpectedly 

 disclosed to the perusal of modern men. 



The earliest system, that of the Gneiss and Mica Slate, ap- 

 pears to be Azoic, or entirely devoid of the remains of living 

 things. This would seem to indicate that, at the time when 

 these rocks were forming, no living things existed on the face 

 of the globe. It is remarked, however, that many of these 

 rocks have been subjected to a degree of heat calculated to 

 metamorphose their physical character, and therefore presum- 

 ably sufficient to have obliterated all trace of any organic 

 remains originally embedded in them. It is also observed 

 that, if, as has been thought, living creatures have been con- 

 cerned in the production of limestone, then living creatures 

 must have existed in the time of this formation, as it includes 

 several beds of that kind of rock. 1 To the same purport is 

 the fact of ammoniacal products appearing in certain chemical 

 experiments with the primary rocks. And still more decisive 

 were the detection of minute objects supposed to be the cases 



1 De la Beche's Geological Researches, 1834. 



