THE 



GEOLOGICAL MAGAZINE. 



No. LXXXI.— MARCH, 1871. 



OI^IG-I^sTJLX. ^I2,TIOXj:H1S. 



I. — On a Method of Determining the Mean Thickness of the 

 Sedimentary Eocks of the Globe. 



By James Ceoll, of tlie Geological Survey of Scotland. 



VAEIOUS attempts have been made to measure the positive length 

 of geological periods. Some geologists have sought to deter- 

 mine, roughly, the age of the stratified rocks by calculations based 

 upon their probable thickness and the rate at which they may have 

 been deposited. This method, however, is worthless, because the 

 rates which have been adopted are purely arbitrary. One geologist 

 will take the rate of deposit at a foot in a hundred years, while 

 another will assume it to be a foot in a thousand or perhaps ten 

 thousand years; and, for any reasons that have been assigned, the one 

 rate is just as likely to be correct as the other : for if we examine 

 what is taking place in the ocean bed at the present day, we shall 

 find in some places a foot of sediment laid down in a year, while in 

 other places a foot may not be deposited in a thousand years. The 

 stratified rocks were evidently formed at all possible rates. When 

 we speak of the rate of their formation, we must of course refer to 

 the mean rate; and it is perfectly true that if we knew the thickness 

 of these rocks and the mean rate at which they were deposited, we 

 should have a ready means of determining their positive age. But 

 there appears to be nearly as great uncertainty regarding the thick- 

 ness of the sedimentary rocks as regarding the rate at which they 

 were formed. No doubt we can roughly estimate their probable 

 maximum thickness ; for instance, Professor Eamsay has found, from 

 actual measurement, that the sedimentary formations of Great Britain 

 have a maximum thickness of upwards of 72,000 feet; but all such 

 measurements give us no idea of their mean thickness. What is 

 the mean thickness of the sedimentary rocks of the globe ? On 

 this point geology does not afford a definite answer. Whatever the 

 present mean thickness of the sedimentary rocks of our globe may 

 be, it must be small in comparison to the mean thickness of all the 

 sedimentary rocks which have been formed. This is obvious from 

 the fact that the sedimentary rocks of one age are partly formed 

 from the destruction of the sedimentary rocks of former ages. From 

 the Laurentian age down to the present day, the stratified rocks 

 have been undergoing constant denudation. 



VOL. Vni.— NO. LXXXI. 7 



