248 ANKDAL OF SCIENTIFIC DISCOVERT. 



be cloared up after the construotinn of many local chronologies, 

 determined neitlier hy g^^'^logy alone nor by fossils alone, but 

 with their mutual aid and correction. The generalized results 

 of these may, probably will, lead to the discovery of laws which 

 will render possible greater precision in the correlation of strata ; 

 but it seems more likely to anliciiiate this result from increased 

 knowledge of the facts of distribution, than from any rule govern- 

 ing the order of structural change. 



Mr. Iluxley avoids the acceptance of any theory of progression 

 founded on fossil evidence ; and his reasons for so doing well illus- 

 trate the acute criticism given by Lyell (" Antiquity of Man," p. 

 40/)). Since this essay was written," Sir W. Logan's discovery, in 

 the lower Laurcntian series of Canada, of the Eozobn Canadense, 

 and the subsequent discovery of a similar organism in Connemara, 

 necessitate some modification of the statement that the Protozoa 

 and Coelenterata are unrepresented in the lowest British rocks. 

 That fossil seems to be structurally allied to both sub-kingdoms. 

 The negative evidence it gives does not seem to allect the progres- 

 sion argument. We are justified in assuming it to represent a 

 small part only of the life, to which the enormous mass of lime- 

 stone containing it is due. The other members of the series, of 

 which we have no evidence for assuming this to be the first, exist, 

 if at all, in some unexplored i-egion, but more probably are lost to 

 us by metamorphosis and resorption into the interior of the earth. 



LENGTH OF GEOLOGICAL PERIODS. 



All the facts of geology tend to indicate an antiquity of which 

 we are beginning to fonn but a dim idea. Take, for instance, one 

 single formation, — our well-known chalk. This consists entirely 

 of shells, and fragments of shells, deposited at the bottom of an 

 ancient sea for away from any continent. Such a process as this 

 must be very slow ; probably we should be much above the mark 

 if we were to assume a rate of deposition of ten inches in a cen- 

 tury. Now, the chalk is more than 1,000 feet in thickness, and 

 .would have required, therefore, more than 120,000 years for its 

 formation. The fossiliferous beds of Great Britain, as a whole, 

 are more than 7,000 feet in thickness, and many, which with us 

 measure only a few inches, on the Continent exjjand into strata 

 of immense depth ; while others of great importance elsewhere 

 are Avholly wanting with us, for it is evident that during all the 

 diti'ereut periods in which Great Britain has been dry land, strata 

 have been forming (as is, for example, the case now^) elsewhere 

 and not with us. Moreover, we must remember that manj^ of the 

 strata now existing have' been formed at the expense of older 

 ones ; thus, all the'flint gravels in the south-east of England have 

 been produced by the destruction of chalk. This again is a very 

 slow process. It has been estimated that a cliff 500 feet high 

 will be worn away at the rate of an inch in a century. This may 

 seem a low rate ; but we must bear in mind that along any line of 

 coast there are comparatively few points which are suffering at 

 one time, and, that even on these, when a fall of cliff has taken 



