GEOLOGY AND EVOLUTION. 369 



of volcanic or igneous rocks in Britain is given by Professor 

 Ramsay as 72,584 feet, or, as Mr. Darwin has put it, " very nearly 

 thirteen and three-quarters British miles. Some of the formations," 

 continues Mr. Darwin, " which are represented in England by these 

 beds, are thousands of feet in thickness on the continent Moreover, 

 between each successive formation we have, in the opinion of most 

 geologists, blank periods of enormous length." 



Now, it is these " blank periods " which certainly affect, in the 

 plainest fashion, all questions concerning the operation of biological 

 change in its relations to time past. It is necessary here to bear in 

 mind a few elementary geological axioms, such as the fact that only 

 those rocks (the aqueous or stratified rocks) which have been formed 

 in water contain fossils; and that the igneous, or volcanic, rocks 

 cannot be expected, from the mere fashion of their formation, to 

 present any traces of past life. Thus it is clear the aqueous, or 

 fossil-bearing rocks must have been formed either in the beds of 

 oceans or in shallow water along coast lines, or in lakes, or at the 

 mouths of rivers, the materials for these rocks being supplied by the 

 wear and tear of previously existing formations. The nature of the 

 included fossils is often the best guide to the exact site wherein the 

 soft materials were deposited to form the strata of future epochs. 

 Thus the presence of fossilised corals, star-fishes, and marine forms 

 of fishes would indicate that the formations containing these fossils 

 had been deposited in a sea-bed ; just as the discovery of fresh- 

 water shells and plants in another series of strata would show that 

 these latter beds were of lake origin ; or, as a mixture of fresh-water 

 and marine forms would suggest that the strata had been deposited in 

 brackish water. 



The distinctness of any two or more series of strata is inferred 

 by the geologist when he discovers differences in their included 

 fossils, whilst he also possesses a criterion of their distinctness in 

 their mineral characteristics. Now, each group of stratified rocks is 

 more or less clearly characterised by possessing certain characteristic 

 fossils, which in some cases may be absolutely confined and limited 

 to the one group of formations, never passing into any other series ; 

 or may, on the other hand, extend from one group through another 

 series, or even through several successive formations. But we are 

 accustomed to note that certain fossils specially characterise each 

 formation, and are characteristic of that formation, even when they 

 may have been slightly developed prior to its period, or have passed 

 beyond it into the next epoch. The comparative distinctness of the 

 fossils in the several formations was formerly explained on the 

 assumption, that at the close of each period the forms of life dwel- 

 ling therein were extinguished and killed off by some sudden 

 catastrophe. The life of the succeeding period was supposed to be 



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