414 



NATURE 



{_Sept.li, 1877 



exist in a comet's head could give us the hydrogen 

 spectrum which was discovered with such richness in 

 the Nova, and which is represented in the spectra of most 

 nebulee. 



The A^i97'(? now exists as a nebula so far as its spectrum 

 goes, and the fact not only goes far to support the view I 

 have suggested as against that of Zollner, but it affords 

 collateral evidence of the truth of Thomson and Tail's 

 hypothesis of the true nature of nebulte. 



The nebular hypothesis in its grandeur and simplicity 

 remains untouched by these observations ; the facts so far 

 from being in direct opposition to it help us, I think, all 

 the better to know exactly what a nebula is. 



There is another point of extreme interest to the spec- 

 troscopist if we accept the bright line observed in the star 

 by Dr. Copeland and others to be veritably the chief 

 nebula line. 



It is clear from Dr. Vogel's diagram (given in last 

 week's Nature) that this line brightened relatively with 

 each decrease in the brilliancy of the hydrogen lines. On 

 December 8, 1876, it was much fainter than F, while by 

 March 2, 1S77, F was a mere ghost by the side of it. On 

 any probable supposition the temperature must have been 

 higher at the former date. 



Now it is well knov<n that within certain limits the 

 lines in the spectrum of a compound body get brighter 

 with decrease of temperature, because at the higher one 

 the compound almost entirely ceases to exist as such, and 

 we get the lines of its constituents. It is a fair theory then 

 to suggest that the famous nebula line may belong to a 

 compound. Nay the fact as it stands alone further points 

 to the possibihty that the compound in question contains 

 hydrogen as one of its constituents. 



J. Norman Lockyer 



Craig Dhu, Kingussie, September 10 



THE GLACIAL GEOLOGY OF ORKNEY AND 

 SHETLAND 



NO one can claim to speak with more authority on 

 matters Orcadian than Mr. Laing, and few men are 

 better fitted to judge of evidence and probabilities. His 

 interesting letter (see p. 41 8 of this number of Nature) 

 calls attention to certain points which he regards as 

 affording a crucial test of the value of some contending 

 hypotheses in geology. 



He asserts (i) That there is no evidence that the Orkney 

 and Shetland Islands have ever participated in the general 

 glaciation of Britain. (2) That these islands contain no 

 raised beaches or marine terraces to prove any alteration 

 of the relative levels of sea and land. 



I. It would indeed be extraordinary on any hypothesis 

 that no traces of glaciation should exist in Orkney. 

 Could it reasonably be supposed that at a time when 

 " the adjacent islands of Great Britain and Ireland " lay 

 under a deep mantle of snow and land-ice which pro- 

 truded even from the opposite shores of Caithness, these 

 northern islets enjoyed a happy immunity from the cold 

 which sealed up the more frigid south .' I am afraid that 

 on the contrary we must believe Orkney to have been in 

 as evil case as its neighbours, no matter even if it should 

 have succeeded in subsequently divesting itself of all 

 traces of its wintry garb. It will not be necessary to 



discuss the bearing of Mr. Laing's facts upon any rival 

 geological doctrines if it can be shown that the facts 

 themselves do not exist. He courteously invites examina- 

 tion and disproof, and I think with all deference to him 

 that I can point to evidence which when he next revisits 

 his county will satisfy him that Orkney is no exception to 

 the general glaciated condition of Scotlind. 



I have twice visited Orkney, and each time was too 

 intent upon the curious history of the Old Red Sandstone 

 of that region to have time to note all the features bear- 

 ing upon the glaciation of the islands. But these features 

 were too striking to escape notice, and I find in my note- 

 books and on my map records of the observations jotted 

 down at the time. So far from there being, as Mr. Laing 

 asserts, no trace of ice-work among these islands, I found 

 them to be well glaciated and to contain in particular, 

 excellent illustrations of (i) j-oches moutonnecs, (2) 

 boulder-clay, and (3) valley-moraines. 



1. Mr. Laing mentions the granitic axis which runs 

 north from Stromness. When he has occasion to cross 

 it again, gun in hand, let him stop here and there on the 

 exposed hummocks and he will find them admirably ice- 

 worn and striated. Well-preserved surfaces of this kind 

 overlook the wild cliffs of Yesnaby, and others, of equal 

 clearness, occur on the slopes behind Stromness. But 

 further examination will show him that these markings 

 are not confined to the hard granite and gneiss. Thus 

 on the roadside at the south-east end of the Lake of 

 Stennis, beautifully striated flagstones may bo seen, the 

 stria; in all these cases running north-west and south- 

 east, as if produced by a movement from the latter 

 quarter. Nay, even among the soft yellow sandstones of 

 Hoy, well smoothed and striated surfaces may be noticed 

 on the summit of the cliffs near the Old Man, at a height 

 of 600 or 700 feet above the waves of the Atlantic. 



2. Unmistakable boulder-clay occurs in Orkney. It 

 is not generally or thickly spread" over the surface, as in 

 the lowlands of Scotland, but rather, as in Caithness, lies 

 here and there in hollows, the rest of the surface of the 

 islands being covered with a thin argillaceous soil, 

 derived, as Mr. Laing points out, from the decay of the 

 underlying flagstones. A thick mass of this boulder-clay 

 lies on the north-west side of Shapinsha, another in the 

 sheltered hollow of Kirkwall Bay, and a third forms a 

 notable feature on the north coast of Flota. Mr. Laing 

 cannot but be familiar with these and other localities, and 

 he probably refers the deposits there to disintegration of 

 the rock underneath. Of course the boulder-clay consists 

 here, as elsewhere, mainly of the debris of the rocks 

 below, and as these rocks are flagstone;, breaking up into 

 sharp-edged fragments, the stones in the clay are very 

 commonly more or less angular. If, however, he finds, 

 as he will assuredly do, that many of the stones are well 

 polished and striated along their major axis, he may be 

 satisfied that the deposit is a glacial one. 



3. So far, the evidence which I have adduced shows 

 that the Orkney Islands participated in the general wide- 

 spread glaciation of the adjacent mainland. But we ni:iy 

 believe that in so northern a locality, if the form and 

 height of the ground in any manner permitted, the linger- 

 ing snows would still form glaciers on the hills, though 

 they had retreated from the lower grounds. Now there 

 is only one mass of high ground in Orkney — the island 



