EPISODE:
EDINBURGH, 1812
"We are never more disposed to give credit to a philosophical system, than when we meet with a case of its successful application, unknown to the author, or containing circumstances which he had not taken into account when he formed that system."
Sir
James Hall (1761-1832)
studied chemistry under Joseph Black at the University of Edinburgh and knew the
philosopher-scientist James Hutton well. Hall is best-known for his experimental
work on the origin of igneous rocks, but he took an active interest in other
aspects of geology.
In a paper given to the Royal Society of Edinburgh in 1812 ("On the Revolutions of the Earth's Surface") Hall ingeniously and imaginatively combined Hutton's vision of the rock cycle, powered by the "expansive force" of Plutonic heat, with a solution to problem of Alpine erratics together with several other open questions of the time.
Hall was impressed at accounts of great devastation by tidal waves following earthquakes at Lisbon and Cadiz (Spain) in 1755, and Callao (Peru) in 1687 and 1746. He reasoned that displacement of the ocean floor could cause such waves (in this he very accurately explained tidal waves, and confirmed his idea experimentally by setting off an explosion under water). Displacements could be caused by expansion within Earth's interior – the same "expansive force" that Hutton has reasoned was the cause of uplift and folding. Hall termed these waves, "diluvial waves." The term "diluvial" was then associated with the Genesis Flood but, as a Huttonian, Hall thought they would have occurred repeatedly throughout Earth's history. The erratics were evidence only of the most recent upheaval.
In the case of the Alpine-Jura erratics, Hall argued that "a wave washing over these high alpine valleys in summer, would float and carry off all the ice in the glaciers . . . and, along with the ice, all the blocks of stone imbedded in it, or attached to it in any way . . . this load would find its way through every opening, and would in a particular manner flow through those depressions [valleys, passes], which at this day . . . afford a view of the snowy summit of Mount Blanc from certain places on the face of Jura where these blocks abound." This mechanism, operating in the mountains of Scandinavia, could also provide a source for erratics in the Baltic region. [Painting by Theodore Rousseau, 1864, looking over Lake Geneva from the Jura towards Mont Blanc: http://www.19thc-artworldwide.org/spring_09/articles/gr/newd-kell_06b.jpg].
But Hall was
not finished.
He also considered
the origin of the depression in the Rhone valley occupied by Lake Geneva, for
which there was no obvious cause (certainly not the existing river that flows in
and out of it). The
initial uplift could cause "a number of rents would be formed, in the hard outer
crust, which would widen upwards as the heave advanced, thus forming the
rudiments of valleys of every degree of wideness, from the narrowest ravine to
the most open vale." This could apply to the origin of valleys in general.
He also incorporated work being done on the most recent fauna and its extinction: ". . . the bones of animals, natives of the hottest climates, which are found over a frozen region of vast extent in the Russian empire . . . an enormous torrent had transported their carcasses across the ridge of Tartary." Here, he acknowledged the work of the German savant, Peter Simon Pallas (1741 - 1811).
Then Hall
turned to other regions. Surely the effects of these waves ought to be found
everywhere? "If such tremendous agents did in reality exert their influence in
the Alps, it is not conceivable that other
countries, situated lower, and
composed of materials much more frail, could have been spared. . . . looking
for traces of the same agents in this country, I found them in abundance . . .
in the neighbourhood of Edinburgh; and the
circumstances of this sort which I
have met with, both on a large scale and in detail, seem to afford more precise
information as to these events than has hitherto been furnished by the alpine
phenomena."
One class of feature among several in the Edinburgh area he ascribed to a "diluvial wave" was represented by Edinburgh Castle. The castle itself sits on a volcanic pipe (the rocky mass under the castle). Hall: "The rock itself . . . consists of one of the most complete and uniform masses of whinestone that is known in this country. Its form is rudely cylindrical, and from it the ridge upon which the Old Town stands, composed partly of deposites, and partly of protected strata, extends, gently sloping, for about a mile to the eastward, from the Castle to the Abbey of Holyroodhouse, where the tail terminates."
http://www.rcahms.gov.uk/image-details.html?img=145&ref=SC795473
(Geikie, A. 1877 p.18)
The "crag and tail" is flanked by valleys on both sides -- however, the current drainage of the area is at right-angles to this. Other, numerous crag and tails in the region (such as the twin Blackford and Braid Hills, 2 miles south of the castle) have identical orientations.
Elsewhere in the Edinburgh area, Hall ascribed scratches, grooves and gougings to the abrasive action of these waters – all of which had the same orientation as the castle crag and tail and other examples.
Hall argued: "Such an arrangement cannot have been the work of the diurnal waters produced by our common rains; for the course of such waters, flowing by the action of gravity, and guided by the general slope of the country . . . ought to have produced depositions nearly at right angles to those under consideration. . . . Now, [if anything] the diurnal operations are everywhere found in the act of corroding and altering the forms here alluded to; but they are nowhere seen to produce them. This class of facts, on the other hand, all conspire in giving probability to the hypothesis of a diluvian wave, which affords an easy explanation of all the large features of this country."
Hall's was arguably the most complete, integrated account of hydrologic, geologic, and biologic data to date. His mechanism was speculative only with regard to the magnitude of the wave(s), which would have to be several thousand feet high in the Alps.
QUESTIONS:
REFERENCES
Geikie, A. 1877 The Great Ice Age and its antiquity in relation to man (D. Appleton & Co. New York)
http://books.google.com/books?id=zjQaeq03ikIC&pg=PR3
Hall, J, 1812 "On the Revolutions of the Earth's Surface, being an account of the diluvian facts in the neighbourhood of Edinburgh" Part 1 Transactions of the Royal Society of Edinburgh, v. 5, 1812. p. 139-167; Part 2 1812, v. 7, p.169-211
http://vinyl2.sentex.ca/~tcc/GT/Hall.html
Also:
http://www.fettes.com/lothian/Corstorphine%20Hill.htm
http://www.fettes.com/lothian/diluvian.htm
