- 
566 The American Naturalist. [June, 
one of these obstacles very frequently interferes, in a 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 expendi- 
ture 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. Ihave not the slightest desire to undervalue elaborate mapping, 
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 difficulties in a conclusion 
already attained, and the disclosure of the precise mode in which cer- 
tain effects have been produced. The criticism was historically unjus- 
tifiable because, so far as my knowledge goes, it is a fact that in regard 
to difficult petrological 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 need- 
ful 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: 
‘1. 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, 
“2. That many such groups can be proved to be older than any 
Paleozoic rock. 
“3. That though crystalline schists have often been claimed as 
metamorphosed sedimentary strata of Paleozoic 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. 
7 4. That in certain cases structures exist in the crystalline schists 
which must be indicative of sedimentation, and that in not a few 
TOS a sequence can be detected which must be due to successive 
deposition. Great as modifications resulting from subsequent pressure 
very frequently are, these may often be separated, and the earlier 
record as in the case of a palimpset be deciphered. 
