200 CORRESPONDENCE. 



subject on which I will venture to speak — good sections, sometimes of 

 more than local interest, such as the junctions between formations, are 

 often lost to science for want of the necessary funds to have them photo- 

 graphed while they are fresh, or before they become gi-ass-gi-own. When 

 original obsem'ations are made, and perhaps local discoveries — which, of 

 course, can scarcely be hoped to happen at very short intei-vals — the 

 results are communicated to the local scicutihc society, and bej-ond a 

 necessarily brief newspaper notice they seldom find their way into print, 

 and are soon beyond the reach of reference. Geology, above all subjects, 

 has to depeiad so much on artificial openings being made in the rocks 

 that vigilant observers, who will be ready at all times, and often at no 

 little personal inconvenience, to take advantage of any artificial exposures 

 that may be made, are needed in every district. Of coiu-se vex-y much of the 

 value of such scientific work as is proposed .lep.ends on its thoroughness. 

 Mere flimsy, or "kid glove," observers, who seem to imagine that the more 

 ground they can contrive to cover, no matter how imperfectly, the greater 

 their achievements, should be as far as possible avoided. — J. S. 



6Icaninris. 



A Sturgeon, 8ft. long and 2301bs. in weight, was recently caught in the 

 Estuary of the Severn. This magnificent specimen has been purchased 

 by Mr. Montagu Bi'owne, Naturalist, Birmingham. We understand he 

 purposes mounting it and presenting it to the Binningham Aquarium. 



Geological. — Readers of Mr. J. Shipman's paper in the " Midland 

 Natiu-alist" for January and February last, entitled " Some new Features 

 in the Geology of East Nottingham," in which several important errors 

 in the geological map of the survey were pointed out, will, we feel sure, 

 be glad to leam that Mr. Aveline, the district surveyor of the geological 

 survey, has veiy recently gone over the gi'ouud examined by Mr. Shipman, 

 and wiU shortly issue a corrected map and memoir of this district. 



The Colouring Matter in the Plumage of Birds. — Mr. H. C. 

 Sorby, F.R.S., recently dehvered a lecture on this subject before the 

 Selby Naturalists' Society. He commenced by explaining the cause of 

 colour in general, stating it to be due to the absorption of some of the 

 prismatic colours and the I'eflection of others. White is produced when 

 all the colours are reflected and none absorbed, whilst black is the result 

 when all are absorbed and none reflected. The colours of feathers are 

 due, first, to the presence of a colouring matter called i^igment, which 

 may be extracted and used as a paint ; second, to the reflection of the 

 prismatic colours of light by the peculiar construction of the laminae in 

 the structure of the feathers. Feathers of the first kind are those which 

 show the same colour both by reflected and by transmitted light. The 

 lecturer exhibited a number of water-colour drawings, painted with the 

 pigments extracted from feathers, and observed that in one instance 

 copper was found to be one of the elements in the composition of the 

 colour, which is, perhaps, the only case known in the animal kingdom 

 where copper forms part of the normal structm-e. A connection had 

 been observed between birds having bright coloured plumage and the 

 flowers on plants on which they feed, the colour of tlie flowers apparently 

 being developed in the feathers of the birds, especially in the yellow 

 colour, whilst birds of pi'ey were usually devoid of yellow colouring. 

 Grey was shown to be diluted black pigment. The second kind of 

 colours are those which, hke the iridescence of a soap bubble are caused 

 by the reflection of hght from two surfaces nearly parallel ; examples 

 of this kind are found in the feathers of the peacock, pheasant, 

 humming bird, &c., tlio colours not being caused by pigment, but due 

 to a curious optical phenomenon. 



