Dec. 13. 1877] 



NATURE 



123 



ground for a discussion which I cannot consider it profitable to 

 continue. WiLLIAM B. CARPENTER 



The Glacial Geology of Orkney and Shetland 



Owing to an accident I did not see your number of Sep- 

 tember 13 containing my letter on the glacial geoloj^y of Orkney 

 and Shetland and Prof. Geikie's article (vol. xvi. p. 414), until 

 my return from Scotland a few days ago. Otherwise 1 should 

 have troubled you sooner with a few observations thereon. 



In tbe first place I wish to thank Prof. Geikie for the very 

 courteous manner in which he has referred to the remarks of an 

 outsider who has ventured to intrude on what the Professor has 

 made, to such an extent, his own peculiar province. 



In the next place I am glad to find that upon what was the 

 most important fact in my statement, viz., the absence cf ra sed 

 beaches or other signs of recent elevation of the land in Orkney, 

 Prof. Geikie agrees with me. 



I call this the most important because it bears directly on the 

 theory cf wide-spreid changes in the relative level of sea and 

 land owing to secular causes, such as a change in the axis of the 

 earth's rotation, or in the position of its centre of gravity. If it 

 can be proved that the difference of level, which caused the 

 raised beaches of the south of Scotland, and extt:nded north 

 along the coast of Ross and Sutherland, dies out as we proceed 

 further rorih, and disappears altogether in Orkney and ShetlanI, 

 it is truly a crucial experiment which shows that these raided 

 beaches are due to local elevations of the land, and not to a 

 general s^inking of the sea. 



This is the conclusion to which Prof, Geikie points, though 

 he naturally finds it difficult to understand why the upheaval, so 

 marked in Sutherland, did not affect Caithness and Orkney. 



I believe I can add a few facts which may assist in removing 

 these doubts. 



At one of the places in Caithness mentioned by Prof. Geikie, 

 where the existence of a raised beich mij^ht be possible, viz., in 

 the sheltered Bay, between Freswick and Wick, I believe there 

 is one, though less strongly marked and at a lower elevation 

 than these in similar situations in Sutherland. I allude to a 

 terrace which bounds the links of Keiss Bay, about half a mile 

 inland from the present coast-line. I cannot speak positively, 

 not having seen it for some years ; but my recollection is that 

 it is a perfect miniature reproduction of the terraces round Brora 

 and other bays in Sutherland. If so, it is a positive proof that 

 the elevation of the land died out towards the north, and we 

 might reasonably suppose that somewhere about the line of the 

 Pentland Firth was the neutral axis, on one side of which the 

 land rose, while on the other it fell. 



Be this as it may, the fact is, I think, incontrovertible that 

 Orkney did not share in the southern movement of elevation. 

 This rests not only on the absence of raised beaches, forming 

 terraces, which might possibly have disappeared, but still more 

 on the absence of all traces of marine action, such as pebbles, 

 sand, or shells, on the low plains which must have been 

 submerged. 



I would ask Prof. Geikie to consider whether the single 

 instance cf the Loch of Stennis is not conclusive. If the sea had 

 ever stood twenty or thirty feet higher relatively to the land than 

 it now does, the whole plain up to the hills must have been a 

 sheltered, shallow, inland fiord. 



As the land rose to its present level this must have left not 

 only a terraced beach at the foot of the hills, which might pos- 

 sibly have disappeared (though it is hard to see why it should 

 have done so in such a sheltered situation), but the whole plain 

 must have been a raised sea-bottom, strewed over with pebbles, 

 sand, and shells. These could not have disappeared, and as 

 they are nowhere visible and the plain consists everywhere of the 

 ordinary rock, with a thin mantle of soil resulting from its disin- 

 tegration by ordinary atmospheric causes, I am, I think, justified 

 in assuming it to be proved that Oikney did not share in the 

 recent movement of elevation which affected the rest of Scotland. 

 Now one word as to glaciation. I can assure Prof. Geikie 

 that I do not think for a moment of setting my authority against 

 his, and that if he is right in the instances of glaciation he tells 

 us he has observed in Orkney, so far from being disappointed, I 

 shall be pleased, for it will clear up what has long seemed to me 

 a perplexing anomaly. 



Of course Orkney must have experienced the full rigour of the 

 glacial period, and it is only natural to expect that it should 

 show the same abundant signs of glaciation as the adjoining 

 counties of Scotland. Prof. Geikie will therefore excuse me if 



I still retain a little of that healthy scepticism which is go con- 

 ducive to the establishment of truth, and venture to plead that 

 judgment may ba suspended until there is further evidence. I 

 d'l so mainly because the Professor's own statement is that during 

 his visits to Orkney his atttntion was devoted mainly to the old 

 red sandstone, and his remarks on glaciation weie only inciden'aL 

 Now there are some proofs of glaciation which are so obvious 

 that there can be no m'stake about them, others which may 

 easily be mistaken, and which require close examination by a 

 practised eye directed specially to them, to arrive at a just con- 

 clusion. 



Boulders of foreign rock, perched blocks, rocks unmistakably 

 rounded and polished by the ice plane, are among the former. 

 But strias require yreat practice and careful exami>iatiori to be 

 sure of them in a district of finely laminated sandstones which 

 weather constantly into parallel lines or grooves. Stony clay 

 a^ain, from disintegrated rock, is o'^ten so like boulder clay that 

 it requires close observation to distinguish one from the other. 

 And finally where steep hills have crumbled away and filled 

 up many places in the narrow valleys between them with their 

 dibris, as at Hoy, the appearances are very like those of glacial 

 moraines. 



Now I observe that nearly all the conclusive proofs of glacial 

 action are wanting in Prof. Gi;ikie's enumeration. He has not 

 seen, or heard of anyone who has seen, a single boulder or perched 

 block, or even a single piece of foreign itone in Orkney. 



As regards boulder-clay I would join issue on his instances, 

 taking especially that of Kirkwall Biy, btcm^e it is typical of 

 the other cases and so easily accessible that the facts can readily 

 be verified. 



I believe it to be disintegrated and not boalder clay, for the 

 following reasons : — 



1. The clay is not compact like that of genuine bouHer-clay, 

 but of looser structure, and often clearly mide up of minute 

 splinters of the disintegrated rock. 



2. The stones in the clay are never foreign stones, and are not 

 scattered irregularly, as if shot out into a huge rubbish heap, as 

 in true boulder-clay, but arranged for the most pirt so that the 

 original lines o^ stratification can be followeH. 



3. If the section which resembles boulder-clay be followed up, 

 it will be found to merge insensibly in what is unmistakably 

 the common disintegrated surface .'■oil of the district. 



• There only remains the question of ro.he^ mcutonnSes, and here 

 I speak with the greatest diffidence, for cenainly Prof. Geikie 

 ought to know a great deal better than I whether a hummock of 

 rock is or is not " admirably ice- worn and striated" like those 

 behind Stromness. 



I can only say that I have looked at them often, and they 

 appear to me to be very different from the roches moutonnies of 

 which I have seen so many in Scotland, Wales, and Swiizerlan \. 

 They are not rounded, sm )Oth, and polished, as if planed into 

 shape by some gigantic tool, but simply irregular hummocks of 

 rock, sometimes smooth and sometime rou^h, according to 

 accidents in the bedding and weathering of the strata. So at 

 least they seem to me, and even in the valleys of Hoy, where, if 

 anywhere, there were local glaciers, the sections shown bv the 

 small streams and low coast-line, always, I believe, exhibit the 

 same appearance of sandstone strata, coming at an angle to tha 

 surface, and with their edges not planed off, but passing 

 gradually into surface soil by disintegration. 



Of course I make these statements subject to correction. It 

 may be that I have failed to see things because my eye is not suffi- 

 ciently educated. But when we couple what is, I believe, abso- 

 lutely certain, viz., the absence of the more prominent and 

 obvious proofs of glaciation in the form of boulders and foreign 

 rocks, with the equally certain fact that O.kney was an excep- 

 tion to the general rule of recent elevation, I think Prof. Geikie 

 will admit that the interests of science will be promoted by any 

 remarks which miy lead to reasonable doubts, and therefore to 

 conclusive investigation, as to the fact whether Orkney does or 

 does not give proof of having been covered by a great polar ice- 

 sheet during the glacial period. 



36, Wilton Crescent, S.W. 



S. Laing 



Explosions 

 I HAVE been waiting to see if Mr. Galloway's paper on 

 "Explosions in Mines" published in Nature, vol. xvii, p. 21, 

 would lead to any conespondence. Your readers may be in- 

 terested in an incident reported to me Jjy the late Dr. Bottinger, 

 of Messrs. i^llsopp's brewery, Kurton-on-Trent, 



