GEOLOGY. 249 



place, the fragments serve as a protection to the coast until they 

 have been gradually removed by the waves. The Wealden Val- 

 ley is twenty-two miles in breadth, and on these data it has been 

 calculated that the denudation of the Weald must have required 

 more than 160,000,000 of years. — Lubbock's Pre-Historic Times. 



SUCCESSION" OF DEPOSITS. 



A paper was read before the Cambridge Philosophical Society, 

 by Mr. H. G. Seeley, F.G.S., on the laws which have determined 

 the distribution of Life and of Rocks. The author observed that 

 in all denudation, whether marine or subaerial and fluviatile, the 

 crystalline rocks which underwent this process, were again de- 

 posited in the following order: (1) Sand, (2) Clay, (3) Lime- 

 stone ; the second overlapping and appearing at the junction to 

 be supei-jDosed on the first, and the third on the second. Hence 

 these desjjosits, which at first sight appeai-ed to be successive, 

 might in reality be contemporaneous. Again, deposits were com- 

 monly assumed to be contemporaneous when they contained the 

 same fossils ; but upheaval and depression would cause the fauna 

 of any locality to move, so that remains of the same species 

 might be dejiosited necessarily in different deposits. He also 

 considei'ed sjiecies to be re-transmutable, and to be affected by 

 the physical conditions under which the animal was living. Thei'e- 

 fore, he maintained that strata could not be identified by these 

 means, but by discovering the j^hysical conditions under which 

 they were deposited, and by other methods. 



EPOCHS OF DEPOSIT OF GOLD. 



Mr. David Forbes, in the " Geological Magazine," has a paper 

 on the geological periods at which gold has made its appearance 

 in the crust of our globe. He designates the two epochs of auri- 

 ferous impregnation as, 1. The older or auriferous granite out- 

 burst. 2. The younger or auriferous diorite outljurst. The first 

 occuiTcd some time between the Silurian and Carboniferous 

 periods. The gold formations belonging to this period present 

 themselves in Australia, Bohemia, Bolivia, Brazil, Buenos Ayres, 

 Chili, Cornwall, Ecuador, Hungary, Mexico, New Grenada, Nor- 

 way, Peru, Sweden, Ural, Wicklow; and also such deposits of 

 gold as are found intruded as quartz nodules and veins, as if 

 interstratified in the Cambrian and Silurian systems, which he 

 believes to have been rendered auriferous solely from their prox- 

 imity to invisible or now superficial granites. The newer outburst 

 cut through strata containing fossils of decided Post-oolitic forms, 

 and possibly may be as late as early Cretaceous. If Mr. Forbes 

 is correct with respect to this comparatively recent creation, so to 

 speak, of gold, we may hope that, whatever is the case with coal, 

 the supply of gold may possibly be inexhaustible ; as there seems 

 no reason why fresh "outbursts" of the igneous diorite should 

 not recur at any period. 



