Page:Science vol. 5.djvu/97

 FRIDAT, JANUABY 30, 1886.

COMMEUT AND CRITICISM. ^TsE LACK of truly demoustrative evidence, I the solution of certaiu geological problems that have beeu regarded as settled for years before they are overthrown, fiiida uen- illus- tratioD in the remarkable results lately aii- niDced by the geological survey of Great bftain. which form the subject of a paper by e of the contributors to our paper this week. The conclusion, that now seems to lie 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 rorme<l part of a continuous series with the underlying sedi- mentary strata, and hence were tliought to like the latter, of sedimentary origin ; this conclusion held until an abrupt tatnct-liae, that bad previously escaped found between the dissimilar formations. Preciaely 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 nicthml of disproving the error in such a case is now too well known, and in this example is too easily applied, to allow any general acceptance of so visible a mistake. In the same way. the essential element in the observations wbicli Murchison and Geikie considered conclusive as establishing the Silurian age of certain High- land schists and gueisaea was the continuity of the aeries, without break by unconforrait)- or dislocation, from the underlying fosailiferous ^^^ds to the overlying crystalline members ; and, ^^^h the strength of their re|)ort to tliis effect, ^^^b Silurian age of the now crystalline masses ^^Bb been for years accepted bv many geolo-

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��Now, it appears that these early observations wei-e too hasty. Examination by more scep- tical observers, and recent deliberate official studies mapjied on the ideal scale of six inches to a mile, discover a raoal 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 fosailiferous and cryaLiUiue beds will be examined over again in the lightof these fruitful discoveries. These overriding Scotch gneisses may thus prove to be the connecting-link between the well-estab- lished alpine inversions that lay the fundamen- tal gneiss on mcaozoic limestone, as on the northern chffa of the Jungfrau, and the still unsolved mystery in Norway, where crystalline sehiats 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 Tdruebohm'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

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