T. Mellard Reade— Geological Time. 99 
the case clearly from the point of view of the present paper, if a 
series of borings were made over the great continents down to the 
base of the Cambrian, so as to enable us to estimate the mean 
thickness of all the sediments laid down since the beginning of the 
Cambrian, we should get the figures we require for our calculation. 
I am afraid that this is not likely to be done, at all events at 
present, either for scientific purposes, or by the wildest company 
promoter. We must therefore be content to adopt a minimum 
thickness. Considering that we have colliery sinkings all in one 
formation over half a mile deep, and numerous borings of half a 
mile, and one of a mile, through sedimentary rocks in various places, 
without reaching the Cambrian base, few geologists will cavil if we 
take the average thickness of the earth’s sedimentary crust down to 
the Cambrian base at one mile. , 
Second as to area. In estimating the area of the sedimentary 
rocks, we are met with the difficulty that we have no actual proof 
of their extent under the ocean bed. There are two extreme, and 
some intermediate, opinions held by modern geologists. The one 
claim that sedimentary rocks underlie all the oceans, the other that 
they extend only a limited distance from the present coast-lines. It 
is, however, clear that the most strenuous advocate of the permanence 
of Ocean Basins will admit that there is sediment of one kind or 
another over the whole ocean bottom, though he is led from various 
considerations to believe that it has never formed land. Still it 
is sediment in part to be legitimately claimed as entering into our 
calculation of geological time. 
For the purposes of this paper I will, however, take the bulk of 
post-Archzan sediment in and under the ocean as equal to that on 
the present land area of the globe, or, to state it in other words, the 
post-Archzan sediments of the whole globe are equal to the present 
land area two miles thick. 
Rate of Erosion of Igneous Rocks. 
There appears to be a concensus of opinion that one foot in 3000 
years is a fair estimate of the mean rate of sub-aerial erosion over 
all land areas throughout all geological time. I am prepared to 
adopt these figures for Archean and igneous areas also, though they 
are probably considerably in excess of what occurs now, the mean 
rate of erosion of igneous and gneissic rocks being Jess than the 
mean rate of erosion of sedimentary rocks, as is proved by their 
usually standing up as bosses amongst sedimentary rocks which have 
been stripped from them by denudation. 
CALCULATION OF THE TIME WHICH HAS ELAPSED SINCE THE CoMMENCE- 
MENT OF THE CAMBRIAN. 
The calculation now is a very simple one. The mean area of 
denudation throughout post-Archzean times being taken as one-third 
the entire land areas of the globe, the bulk of the post-Archzean rocks 
being expressed by the land area of the globe two miles thick, and 
the rate of denudation one foot in 3000 years, the time of accumula- 
