452 
NATURE 
[AZarch 3, 1870 
and fishes. In the Colleges, there are laboratories at 
St. John’s, Sidney Sussex, and Downing ; and we believe 
that Trinity College contemplates establishing one. 
(3) INDUCEMENTS.—The degree of B.A. may be ob- 
tained in Natural Sciences. An examination in Honours 
was instituted in 1851; in 1861 the regulations were 
revised, and the successful candidates were declared en- 
titled toa degree. Ninety-five students have passed this 
examination in the nine years since the alteration. A 
candidate for an ordinary degree may also select for the 
subject of his third or final examination one of the fol- 
lowing subjects: Chemistry, Physics, Geology, Botany, 
Zoology. In the Colleges: Clare gives annually a scholar- 
ship, value 507.; Caius two, value not stated, one for 
Chemistry, the other for Anatomy ; Christ’s has lately 
oftered scholarships, from one to four in number, and from 
30/. to 707, in value, according to the merit of the candi- 
dates ; St. Peter’s gives annually one of the value of 60/7. ; 
St. John’s gives annually an exhibition of 50/7. for three 
years to students commencing residence ; this College has 
also just instituted an annual examination in the Natural 
Sciences for its resident students, for proficiency in which 
prizes in books and pecuniary rewards will be given, as in 
the other College examinations; Trinity gives annually 
one foundation scholarship, tenable till the holder is of 
M.A. standing ; Sidney Sussex, two scholarships annually, 
value 40/., with opportunity of promotion, for Mathematics 
or Natural Science ; Downing gives annually at least one 
scholarship, value 4o/, A fuller description of these will 
be found in No. 6 of this periodical, p. 169. 
In looking through the lists of the Natural Sciences 
Tripos, fourteen persons will be found to have been 
elected fellows, but in most cases the candidate has been 
not without distinction in other branches of study. In 
several, however, proficiency in Natural Science was the 
declared cause of the election. 
These statements are made upon the authority of the 
last volume of the Cambridge Calendar, supplemented in 
some instances by personal knowledge. 
Thus much has been done: of what remains to do it is 
perhaps better that one, who is a resident and engaged to 
some extent in the work, should refrain from speaking. 
On this point only I may venture to express my convic- 
tion, that the coldness and even dislike with which 
the study of Natural Science was once regarded here is 
rapidly passing away, that the number of earnest students 
in the various branches is annually increasing, and that 
the University is fully alive to the wants of the age; so 
that, while she can never neglect or forget those old 
paths of Classics and Mathematics in which many of her 
sons have won an almost world-wide reputation, she will 
heartily welcome, and will regard with no less pride, all 
who are among the followers of sciences of a more 
recent date. T. G. BONNEY 
THE MEASUREMENT OF GEOLOGICAL TIME 
Il. 
We have now to consider an entirely distinct set of facts 
_Which have an important bearing on the probable time 
clapsed since the last glacial epoch. Messrs. A. Tylor, Croll, 
and Geikie have shown that the amount of denudation now 
taking place is much greater than has generally been 
\ 
supposed. The quantity of water discharged by several 
rivers and the quantity of sediment carried down by those 
rivers have been measured with tolerable accuracy, and 
allowing for the difference of specific gravity between 
sediment and rock, it can be easily calculated, from the 
known area of each river basin, what average thickness 
has been removed from its whole surface in a year, since 
all the matter brought down by the river must evidently 
have come from some part of its basin. In this way it is 
found that the Mississippi has its basin lowered gqyp of a 
foot per annum; the Ganges, ;; the Rhone, zs; 
the Hoang-Ho, 74; the Po, -4, 
But it is evident that this amount will be distributed 
very unequally over different parts of the basin, according 
as the surface is flat or sloping, whether the slopes are of 
loose soil or of rock, whether the rock is solid or friable. 
The perfectly flat alluvial plains that form a considerable 
part of many river basins, will not only suffer no denudation, 
but will generally receive deposits of sediment during 
floods, and all such flat lands should therefore be deducted 
from the area of the river. Slightly undulating lands, 
especially if well covered with forest, will also suffer 
scarcely any denudation, as is well seen in the case of the 
Rio Negro branch of the Amazon and other black water 
rivers of South America, which hardly carry down any 
perceptible sediment even when in full flood. Again, 
wherever lakes occur, they receive all the sediment from 
the basin above them, which portion should therefore be 
treated by itself, since it contributes no sediment to the 
main river. If we look at a physical map of North 
America we see that a large extent of the Mississippi 
basin consists of alluvial flats and slightly undulating 
prairie, sufficiently explaining its small proportionate 
denudation. Even the Rhone, which has a high rate of 
denudation, flows through a great extent of low lands and 
perfectly flat meadows, while the upper portion of its 
valley which produces most sediment is cut off by the Lake 
of Geneva. In order, therefore, to arrive at any fair 
estimate of the amount of denudation in the upland and 
mountainous portions of the Rhone valley (which is what 
we require for our purpose) we have considerably to 
reduce the area of its basin by taking away the flat lands 
in all its valleys, and considerably to increase the amount 
of sediment by adding all that is now poured into the Lake 
of Geneva. We shall probably not be far wrong in adding 
one third to its denuding powers on these grounds, which 
will lead us to the startling fact that the Rhone basin is 
being lowered at the rate of a foot ina thousand years; but 
even this is considerably less than in the case of the Po. 
Mr. Croll takes the Mississippi denudation of a foot in six 
thousand years as a measure for that of Europe ; but for 
reasons above stated I conceive this to be quite out of the 
question, and I maintain, that if we are to use his measure 
of denudation for any practical purpose, we must apply 
that of European rivers to European phenomena, that of 
Alpine rivers to Alpine phenomena, and must further make 
the necessary corrections for alluvial flats and intercepting 
lakes. 
Mr. Croll and Sir Charles Lyell were at first both 
inclined to adopt the period of high excentricity which 
occurred from 750,000 to 950,000 years ago as that of the 
glacial epoch, but Mr. Croll, in consideration of the proofs 
of rapid denudation above given, now believes that the 
