188 PROF. T. G. BONNET ON CRYSTALLINE SCHISTS AND THEIR 



out fear of contradiction that a very elaborate petrographical map- 

 ping of the Alps is impossible ; for the most painstaking and con- 

 scientious of surveyors must assume much that is incapable of 

 demonstration. A very large part of the whole area is concealed by 

 snow, glaciers, debris, pasture, forest ; and some one of these ob- 

 stacles very frequently interferes, in the most provoking way, just 

 at the most critical point. Further, no small amount of the rock 

 which is visible can only be regarded from a distance. Many a 

 cliff, many a ridge, is inaccessible, and the examination, even of 

 every point which it would be possible to reach, would require the 

 expenditure of such an amount of time, that I am certain it 

 never has been, and believe that it never will be done. 



But further, the criticism, in my opinion, was scientifically unsound 

 and historically unjustifiable. Scientifically unsound, because very 

 commonly the most important problems which are presented by the 

 crystalline rocks receive a decisive answer from one or two sections 

 only*. I have not the slightest desire to undervalue elaborate map- 

 ping, but we must be careful not to treat it as a fetish, as though it 

 were the only means appointed for the discovery of geological truth. 

 Its results more commonly are the removal of minor difiiculties in a 

 conclusion already attained, and the disclosure of the precise mode 

 in which certain effects have been produced. The criticism was 

 historically unjustifiable, because, so far as my knowledge goes, it is 

 a fact that in regard to difficult petrol ogical questions, infallibility 

 has not been found to reside with the makers of geological maps. 



My work, both in the Alps and in other regions, which has been 

 carried on with a definite object and a fairly clear idea as to the 

 needful evidence, has led me to the following conclusions, which, 

 though they have been already expressed, I will venture to repeat 

 for the information of the reader. 



(i.) That a group of truly crystalline schists is always more 

 ancient than any rock to which, on the evidence of fossils, a date 

 can be assigned. 



(ii.) That many such groups can be proved to be older than any 

 Palaeozoic rock. 



(iii.) That though crystalline schists have often been claimed as 

 metamorphosed sedimentary strata of Palaeozoic or Mesozoic, if not 

 of Tertiary age, the evidence in support of this claim has hitherto 

 always broken down on careful examination, and in not a few 

 instances has proved hardly worthy of the name. 



(iv.) That in certain cases structures exist in the crystalline 

 fjchists which must be indicative of sedimentation, and that in not 

 p. few instances a sequence can be detected which must be due to 



* Por instance, whether an igneous rock be interbedded or intrusive may be 

 an open question in every section but one. Whether a group of slates forms 

 part of a continuous series with a group of schists may be uncertain in most of 

 the sections, but may be settled in the negative by tlae discovery, in a single 

 section, of a conglomerate or breccia of the latter in some member of the former. 

 I do not suppose that this statement will be questioned by any one familiar with 

 this department of geology ; but if it were, I could fill a page readily with 

 examples of the proof or disproof of a theory by a single section. 



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