258 Dr. C. Callaicay — On Comparative LitJwlogfj, 



III. — A Plea for Comparative Lithologt. 

 By C. Callaway, D.Sc, F.G.S. 



IN a short series of papers in this Magazine i on How to worTc in 

 the Archcean Rocks, I briefly set forth the principles of correla- 

 tion which seemed to me available, and pointed out the precautions 

 which should attend the application of each test. I discussed the 

 evidence from (1) Organic remains ; (2) Order of superposition ; 

 (3) Inchided fragments ; and (4) Mineral composition. The last- 

 named criterion has the widest application, and therefore it was 

 necessary to define its scope with great care. I showed that its value 

 largely depended upon accessory considerations. I insisted that 

 rock-groups should be compared as a whole, that the degree of meta- 

 morphism was an important factor in the evidence, that a valuable 

 auxiliary test was to be found in similarity of succession, that the 

 proof of a Pre-Cambrian age enormously reduced the chances of 

 error, and that, in some cases, the origin of the deposits was a useful 

 aid in correlation. I recall attention to these points, because, at a 

 recent debate ^ in the Geological Society, some of my critics seemed 

 to believe that I had been attempting to determine the age of forma- 

 tions by the comparison of a few hand- specimens. They omitted to 

 take fully into account the subsidiary evidence which has been accu- 

 mulated during the last decade. But even if we entirely ignore 

 this mass of proof, it is surely of interest to point out lithological 

 resemblances between rock-systems, even if we are not prepared to 

 correlate them. Twilight is better than absolute darkness. We may 

 say that the Pebidian of Hicks is like the Lower Taconic of 

 Emmons, without asserting the contemporaneity of the two groups ; 

 and then leave the lithological hint at the mercy of the evolutionary 

 laws which will either destroy it or develop it into a theory. 



In the debate to which I have referred, it was suggested that as 

 attempts to correlate ordinary sediments by means of mineral 

 characters had broken down, and similar efforts in connection with 

 igneous rocks were sharing the same fate, it was useless to apply the 

 lithological test to the crystalline schists. This argument is not 

 without apparent force, and requires careful analysis. 



It is quite true that amongst Post-Archaean strata we do not pay 

 much attention to mineral composition ; but the reason for our com- 

 parative neglect is not that we love lithology less but fossils more. 

 If, however, organic remains were to vanish out of the earth's crust, 

 the geological record would not become an utter blank. We should 

 venture to identify the Chalk on both sides of the Straits of Dover, 

 without thinking it necessary to run a tunnel to prove absolute con- 

 tinuity ; and if some bold lithologist were to refer Caen stone to the 

 age of our Oolite, we should not brand him as a mere dreamer. But 

 each case would be determined on its own merits, and it would be 

 impossible to establish even an empirical law of correlation. 



The objection of my critics proceeds upon the assumption that 

 there is the same absence of law amongst the Archeean rocks. It 



1 Aug and Sept., 1881. 



2 On my Donegal paper, March 11th, see abstract, printed on p. 278. 



