SCIENTIFIC SUMMARY. 
435 
found, and the composition of the metal, they consider, contrasts favourably 
with the so-called tin used for similar purposes in England. We find, how- 
ever, that one sample of the foil which they analysed contained 6-111 per 
cent, of antimony and 0-889 per cent, of arsenic. The metal which coats 
“tin-plate” is more easily attacked and dissolved than is generally supposed. 
Menke detected the presence of 0-1513 gramme of tin in a “canned” pine- 
apple weighing If lbs., 0-0101 gramme of that metal in a tin of lobster, and 
0-0067 gramme in one of apples. 
GEOLOGY AND PALAEONTOLOGY. 
A new Pre- Cambrian formation . — Dr. Hicks, whose establishment of the 
Dimetian and Pebidian formations we noticed last year (P. S. Rev., N. S., 
Vol. I., p. 96), states in a communication to the British Association that he 
has found that the Pre-Cambrian rocks occupy many areas in Wales, and 
that they resolve themselves into three well-marked types, which he regards 
as separate formations, as they prove on examination to be unconformable to 
each other. At St. David’s granitoid rocks occur at the base ; resting un- 
conformably upon these come quartz-felsites ; which again are succeeded by 
the agglomerates, breccias, greenstone-bands, and schists of the Pebidian 
group. In North Wales the same order of succession was observed, but the 
middle, or quartz-felsite group, was more largely developed in Caernarvon- 
shire, and hence Dr. Hicks proposes to name it Arvonian , from the Roman 
name of the district (Arvonia). The name is appropriate, as many of the 
mountains of Caernarvonshire consist of these rocks. Dr. Hicks thus dis- 
tinguishes these Pre-Cambrian formations, namely : — 
1. Dimetian , granitoid gneiss rocks. 
2. Arvonian , quartz-felsites and porphyries (Halleflinta of Torel; petro- 
silex rocks, Hunt). 
3. Pebidian , green and purple agglomerates and breccias, green chloritic 
schists, with massive greenstone bands, talcose schists, &c. 
In these formations the bedding is usually easily recognized, but the actual 
stratigraphical thickness cannot yet be precisely estimated, although, from the 
sections exposed, Dr. Hicks infers that each of them must be many thousand 
feet thick, and their horizontal extension is also evidently very wide. 
Fossil Mammals of South America. — From an examination of numerous 
remains of Mammalia, lately brought to Paris from Brazil and La Plata, 
M. Paul Gervais has obtained some important results. He regards Toxodon 
as nearly allied to the Porcine Mammals, and as probably agreeing with the 
Hippopotamus in its habits ; but although it resembles that great old-world 
Pachyderm in some of its characters, it possesses others which indicate that 
it must have nearer allies which are still unknown. The Solidungula have 
no well-marked representatives from the region of La Plata, except the 
horses known as Hippidia. No remains of Tapirs have been found with 
these ; but a fragment of the symphysis of a mandible, having traces of two 
canines with two incisors between them, seems to indicate an animal resem- 
bling Rhinoceros in this part of its dentition. 
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