SCIENCE. 



FRIDAY, JANUARY 30, 1885. 



COMMENT AND CRITICISM. 



The lack of truly demonstrative evidence, 

 in the solution of certain geological problems 

 that have been regarded as settled for years 

 before they are overthrown, finds new illus- 

 tration in the remarkable results lately an- 

 nounced by the geological survey of Great 

 Britain, which form the subject of a paper by 

 one of the contributors to our paper this week. 

 The conclusion, that now seems to be errone- 

 ous, rested on what may be called the argu- 

 ment from continuity- of deposit. The same 

 argument, involving the same error, was used 

 by Werner nearly a century ago to prove the 

 aqueous origin of his ' floetz-trap.' These 

 old lava-flows apparently formed part of a 

 continuous series with the underlying sedi- 

 mentary strata, and hence were thought to 

 be, like the latter, of sedimentary origin ; 

 and this conclusion held until an abrupt 

 contact-line, that had previously escaped 

 notice, was found between the dissimilar 

 formations. Precisely the same reasoning has 

 been employed in recent years to support 

 the aqueous origin of the old lavas in the 

 Palisades of the Hudson ; but the method of 

 disproving the error in such a case is now 

 too well known, and in this example is too 

 easily applied, to allow an} 7 general acceptance 

 of so visible a mistake. In the same way, the 

 essential element in the observations which 

 Murchison and Geikie considered conclusive 

 as establishing the Silurian age of certain High- 

 land schists and gneisses was the continuity of 

 the series, without break by unconformity or 

 dislocation, from the underlying fossiliferous 

 beds to the overlying crystalline members ; and, 

 on the strength of their report to this effect, 

 the Silurian age of the now crystalline masses 

 has been for years accepted by many geolo- 

 gists. 



No. 104.— 1885. 



Now, it appears that these early observations 

 were too hasty. Examination by more scep- 

 tical observers, and recent deliberate official 

 studies mapped on the ideal scale of six inches 

 to a mile, discover a most peculiar discontinu- 

 ity in the form of a nearly horizontal surface of 

 dislocation, across which the overlying mass 

 has been driven actually for miles from its 

 normal inferior position. Whatever possibili- 

 ties may be discovered elsewhere, the paleozoic 

 date for the metamorphism of the Sutherland 

 crystalline series must now be regarded as in- 

 correct, and the origin of their crystalline tex- 

 ture must be set back into earlier ages. The 

 character of the dislocations thus revealed is as 

 important as the disproof they afford of a seri- 

 ous error ; and the inverted attitude that has 

 been observed elsewhere between fossiliferous 

 and crystalline beds will be examined over again 

 in the light of these fruitful discoveries. These 

 overriding Scotch gneisses may thus prove to 

 be the connecting-link between the well-estab- 

 lished alpine inversions that la} 7 the fundamen- 

 tal gneiss on mesozoic limestone, as on the 

 northern clijffs of the Jungfrau, and the still 

 unsolved mystery in Norway, where crystalline 

 schists seem to overlie the fossiliferous paleo- 

 zoic sediments across wide areas, and thus 

 give an abnormal character to the structure of 

 the mountains, as shown in Tornebohm's sec- 

 tion of the peninsula. 



Then there is the extraordinary measure of 

 ten miles for the horizontal displacement that 

 is accountable for the whole difficulty in the 

 Highlands ; and along with this goes the occur- 

 rence of a number of (so-called) ' reversed 

 faults,' in which the uplifted member has been 

 thrust up an inclined plane. All of this is 

 strong in evidence of the modern view that 

 disordered mountain structures are character- 

 ized less by the gain of height than by the loss 

 of breadth that they have suffered. The almost 



