Nov. 30, 1876J 



NATURE 



97 



them hitherto. P. N. Zealandia is not hermaphrodite. I ex- 

 amined several males, which differ in no essential points in their 

 structure from those of/', captnsis. IJke those of/', capensis, 

 they are less numerous than the female?, and Capt. Ilutton has 

 been unlucky enough not to meet with any among.st the twenty 

 specimens examined by him. The jaws of /'. N. Zealandice are 

 further, I believe, developed just as are those of P. capensis. At 

 least I saw that the earliest stages corresponded, and recognised 

 the first pair of members of ihe embryo in P. N. Zealandiie in 

 the stage in which they are not yet turned inwards to become 

 foot jaws. I have prepared a more extended answer to Capt. 

 Hutton's paper with an account of my own observations on 

 /'. N. ZialandiiC for the Ann. and Mag. of Nat. Hist., but as this 

 cannot probably be published immediately, I should be much 

 obliged if you would insert this reply in Nature. 



H. N. MOSELEY, 

 Naturalist to II. M.S. Challenger 



The Age of the Rocks of Charnwood Forest 



It is no doubt to be regretted that Mr. Woodward, misled by 

 insufficient authority, should have introduced, in his excellent 

 work on the geology of England and Wales, still further con- 

 fusion into the maltreated old rocks of Charnwood Forest, but I 

 doubt whether their age is quite so certain as Prof. Hull seems to 

 think. I fully agree with him that there is not a particle of evi- 

 dence for thtir Laurent ian age, and that their syenites and horn- 

 bltndic granites cannot be correlated with the hornblendic gneiss 

 of the Malverns, but I must demur to his grouping them with 

 the Cambrian rocks of the Longmynds or of Llanberis. The 

 authority of Prof. Sedgwick is great, but it must be remembeied 

 that the term Cambrian with him included far move than in the 

 nomenclature of the Geological Survey, and I am not aware 

 that he ever committed himself to the Charnwood ri cks being 

 equivalent to his Lower Cambrians. Except a slight lithological 

 resemblance of sc me Charnwood rocks to those of Harkch 

 and Llaubeiis, and a still slighter to Lorgmynd rocks, there is 

 really nothing in favour of this special correlation. One point, 

 however, there is which may give some clue to their age, which 

 does not seem to have been much noticed hitherto, probably 

 bicause the facts have been strangely overlooked in the Geolo- 

 gical Survey description of the district. It is that beds of coarse 

 volcanic agglomerate and ash abound among the Charnwood 

 series. Further, the resemblance of the rocks as a whole (when 

 not unusually metamorphosed) is very close to the "green slate 

 and porphyry series" (or Borrowdale rock'^) of the Lake District. 

 Compared with the Welsh rocks, they are far more like those of 

 Cader-Idris than of Llanberis. With these there is scarce any 

 lithological resemblance, but if I mixed my Charnwood collec- 

 tion with those from the other two localities, eipecially the former, 

 I should have great difficulty in separating many specimens. It 

 seems then to me far more likely that this great volcanic activity 

 in the Charnwood district should have corresponded in time with 

 that in the Lake District or with some part of that in Wales, than 

 that it should have happened in the age of the Harlech, Llan- 

 beris, and Longmynd groups, where we have no evidence of any 

 volcanic disturbance. The argument may be summed up thu?, 

 as it seems to me : — The Charnwood rocks are old, so are both 

 the competing groups ; they are unfossiliferous, so are both ; 

 they are cleavtd, io are both ; they contain evidence of great 

 volcanic action, so do the Borrowdale series, and not the Welsh 

 Lower Cambrians. One point for the former. The general cor- 

 respondence of their strike with that of the Borrowdale series 

 under Ingleborough may also perhaps count for something. 



T. G. BONNEY 

 St, John's College, Cambridge, November 25 



Though the discussion of the age of the rocks of Charnwood 

 Forest is not likely in the present state of our knuwle 'ge to lead 

 to any useful result, there are still a few points in Prof, Hull's 

 letter on the subject which seem to call for remark. In the first 

 place the late Prof, Jukes was by no means so strongly in favour 

 of the Cambrian age of these rocks as Prof. Hull states. Prof, 

 Jukes' words, in Potter's (not Porter's) " History of Charnwood 

 Forest " are as follows : — " It is therefore uncertain whether they 

 (the rocks of Charnwood) l^long to the Devonian, Silurian, or 

 Cambrian systems, the probability only being in favour of the 

 latter.'' Secondly, the Cambrian of Sedgwick includes a great 

 deal more than the Cambrian of the Geological Survey, and 

 therefore there is not the perfect unanimity between these two 



authorities that Prof, Hull's remarks would lead us to believe. 

 Thirdly, if lithological resemblance is to go for anything, it may be 

 used directly against the Cambrian age of the rocks. On the 

 western side of the forest we find sheets of crystalline rock and 

 beds of highly altered conglomerates and breccias, which have a 

 suggestive likeness to the lava iljws and ash beds of the green 

 slate and porphyry series of the Lake District I don't say the 

 resemblance proves anything, but it is worth quite as much as 

 the similarity between the slates on the east side of the forest and 

 the slates of Llanberis. Mr. Bonney has also called attention to 

 the fact that the strike of the Charnwood Forest rocks is the 

 same as that of the Volcanic Series in the Lake Country, when 

 that group is last seen. Again, it is far from certain that the 

 rocks of Charnwood Forest are all of the same age. I recollect 

 seeing many years ago some sections (of which I am afraid I have 

 kept no record) that seemed to show that some of the bosses of 

 Dioritic rock near Markfield were older than the slates that 

 surrounded them. If this be so, perhaps these crystalline hills 

 may be the projecting pohUs of a nucleus of similar rock that 

 underlies the whole area, and which may be Laurentian in age. 

 The rocks are not gneiss, but I know of no reason why the equi- 

 valents of the rocks of the Hebrides must be gneiss all the world 

 over ; they are, however, rich in hornblende, and so are the 

 Hebridian rocks. With all these possibdities before us, I am 

 afraid it will be hard to arrive at that enviable state of security 

 which Prof, Hull seems to have been in when he penned his 

 letter, A. H, Green 



I AM pleased to find in Nature, vol. xv., p, 78, a letter 

 from Prof. Hull, with reference to the age of our Charnwood 

 Forest rocks. He writes against their assignment by Mr. H, 

 B. Woodward, in his "Geology of England and Wales," to 

 the Laurentian period (see p. 24). 



But, in fact, as Prof, Hull himself points out, we also find on 

 p, 30 a statement that part of the series may belong to the Cam- 

 brian epoch. 



It would appear that as Mr. Woodward is not personally 

 acquainted with the region, he has endeavoured to give the views 

 of the various authors whom he knows to have written on the 

 subject, and as these are conflicting, and based upon little per- 

 sonal work, it is no wonder that he has been led astray. 



I do not think sufficient importance has been attached to the 

 study of this isolated outcrop of old rocks. We can trace its 

 continuation to the south and south-east for a considerable dis- 

 tance, and I would venture to suggest the possibility of a flexure 

 or spur in this direction connecting with the old palaeozoic ridge 

 for which we have lately been fishing in the Wealden. In my 

 " Geology of Leicestershire and Rutland," which will shortly 

 be published, there will be found some fine photographs of the 

 principal quarries and natural outcrops of the Charnwood rocks ; 

 and I have there given the reasons which induce me on the whole 

 to refer the main mass of the rocks to neither Laurentian nor Cam- 

 brian, but to the Silurian period. The evidence is but scanty 

 however, but a balance of probabilities at the best. As to 

 Sedgwick's determination of the region, we must remember that 

 much that he then called Cambrian has since been assigned 

 to Lower Silurian, Wm, Jerome Harrison 



Town Museum, Leicester, November 24 



Minimum Thermometers 



Some time ago a correspondence appeared in Nature (vol. vi, 

 pp, 122, 142, 221) on the subject of moisture deposited in mini- 

 mum thermometers exposed on the grass. As I was at the time 

 much annoyed with this myself I took up every hint I could get 

 in the matter, though I must confess with indifferent success. I 

 tried for a long time india-rubber packing', with sealing-wax, &c., 

 of varying coat?, as advised, but still moisture insmuated itself. 



At last I bethought myself of a cork packing, I cut a piece 

 of cork so as to fit tightly round the neck of the thermometer 

 tube, then inserted the tube and packing into the glass case— 

 the cork packing being about a quarter of an inch long. The 

 exposed end of the cork I covered with two or three coats of 

 asphalte, as used on microscopic slides. At first a slight bubbling 

 was seen through the asphalte, but soon disappeared, and a fine 

 uniform surface at last set in. The thermometer has iiow been 

 in use for several months, and not the least trace of moisture has 

 ever been seen withm the cases, although moisture has been 

 abundant, especially for the last three months. The process is 

 simple enough, and I venture to send it to you, hoping that it 



