249 



ON THE ALLUVIAL DEPOSITS OF EIVERS. 



Bt the Eev. J. D. LATOUCHE, F.G.S. 



One of the most unsatisfactory questions wluch meets the student in 

 Geology, is the determination of the periods or epochs of which it ti-eats. 

 The mind of any one who studies the rocks must almost necessarily entertain 

 this question. For the most part we do but enter upon it to find ourselves 

 completely baffled. Omitting altogether the enormous thickness of strata which 

 underlie those recent deposits containing shells and vegetable matter identical 

 with what at present exists on the surface of the earth— taking these com- 

 paratively modern deposits alone into consideration, the demands made upon our 

 faith in the vast periods of time that even they have taken for their accumula- 

 tion, may well cause us to despair of estimating within the faintest limits of 

 probability those of the sedimentary rocks beneath them. 



As one instance out of many, I may recall the account which Sir C. Lyell 

 gives of Jlr. Horner's attempt to determine the time taken to form the deposit of 

 the Nile. "Mr. Homer suggested to the Royal Society in 1850, that they 

 should have excavations and borings made in the alluvial plain of the Nile, with 

 a view of ascertaining the thickness of the mud which had accumulated around 

 the base of the obelisk at Heliopolis and the pedestal of the statue of Kameses 

 at Memphis, the object being to obtain a chronometric scale by ascertaining 

 what thickness of seduaent had been formed in a given time and by applying 

 that scale for measuiing the antiquity of similar mud previously thi'own down on 

 the site of those monuments before their erection. The most important result 

 was obtained from an excavation and boring made near the base of the pedestal 

 of the colossal statue of Rameses, the middle of whose reign, according to 

 Lepsius, was 1361 B.C. A deposit of 9 feet 4 inches was ascertained, which 

 gives a mean increase of 3^ inches in 100 years. It was then .ascertained by 

 sinking a shaft near the pedestal to an additional depth, that "the thickness 

 of old Nile mud resting on desert sand amounted to 32 feet, and it was there- 

 fore inferred by Mr. Komer, that the lowest layer (in which a fragment of 

 burnt brick was found) was more than 13,000 years old, or was deposited 13,49C 

 years before the year 1850, when the boring was made." 



I have alluded to this one instance out of a great many that might be 

 taken out of Sir C. LyeU's most interesting book, as it is closely allied to the 

 subject of this paper. It is quite evident that in the detritus carried down by 

 streams we have a means of measuring to a considerable extent the progress of 

 Geologic epochs. 



