206 J. G. Goodchild — How to take. Impressions of Fossils. 



felspathoid (leiicite, sodalite, etc.), or a mica, and there may or may 

 not be any undissolved relics of it in the lamprophyre as finally 

 consolidated. 



One other remark may be made in conclusion. In rocks con- 

 taining abnormally large proportions of potash and soda, and having 

 at the same time plenty of alumina, it should not be surprising to 

 find occasionally minerals richer in alkali than the felspars. Novp- 

 at Croiikley, on the Ijanks of the Tees, all the dykes contain a 

 mineral which in thin slices shows hexagonal or quadrangular 

 outlines, with a dark border and nucleus. The sections have no very 

 definite action on polarized light, and seem to be more or less com- 

 pletely converted into obscure decomposition-products. Mr. Rutley 

 regarded the mineral as decomposed garnet, but if it occurred in a 

 phonolite or leucitophyre it would probably be put down confidently 

 as nosean. Witliout expressing an opinion on this point, I will 

 observe that in the most easterly d^'ke, where the mineral in question 

 is most abundant, there occurs another with square contour, bright 

 blue colour, and single refraction, which I can refer to nothing but 

 haiiyne. 



III. — An Improved Method of taking Impressions of Fossils, etc. 



By J. G. Goodchild, F.G.S. 



of Her Majesty's Geological Survey; Lecturer on Palseontology at the Harlot 



Watt College. 



PALiEONTOLOGISTS and others concerned with fossils often 

 have need of some method of taking impressions of fossils, 

 which shall at the same time be simple and efficacious, and shall 

 also be of such a nature as not to cause injury of any kind to the 

 original. Many different processes have been tried, with varied 

 success. The following method has stood the test of application to 

 a wide range of subjects, and has answered its purpose well in the 

 hands of a considerable number of workers : — 



The only outfit required is a small roll of thin tinfoil of ordinary 

 qui^lity, a small plate-brush, neither too hard nor too soft, a bottle of 

 shellac varnish, and some paraflBne wax, with a night light or some 

 such means for melting it. 



If the fossil is not in too high relief, say a fossil fish, or such 

 a plant as a Coal-measure fern, all that is needed is to cut a piece 

 of foil rather larger than the specimen, then to press it gentl}^ with 

 the finger tips at first, into all the larger depressions, beginning at 

 the middle and working outwards towards the edge all round. Then, 

 keeping the fingers extended over the impression, go over the whole 

 thing with the plate-brush, using it as gently as possible, and with 

 only a very slight lateral movement. After a few seconds treatment 

 of this kind an almost exact counterfeit of the fossil will appear — 

 even some of the very finest sculpturings being distinctly visible on 

 the upper surface of the foil. 



"When that stage is reached, the foil should be lifted very gently 

 at each corner so as to free it from any projecting or undercut points. 

 Herein lies the special value of the tinfoil process, inasmuch as 

 this material does not enter the undercut parts as modelling wax, 



