68 SECTIONAL ADDRESSES. 
the globe is to be regarded as a tremendous calamity. This condition 
attained, the universal deluge is within sight geologically speaking, and 
the end of the present order of things must inevitably ensue. Harth- 
quakes, therefore, are not to be regarded as unmixed calamities ; they are 
evidence that the fatal total rigidity has not yet been attained. 
It might be asked if there is any evidence in geological history of an 
approach to a condition of total rigidity or of a tendency in this direction. 
There can be little doubt that Precambrian events were on a scale seldom, 
if ever, attained in later time. Cambrian and Ordovician transgressions 
of the sea were also on a grand scale, but later movements, on the whole, 
seem to have been smaller and more local in their expression, although 
there were notable exceptions as the Tethys sea in Europe, the great inva- 
sion of the Coloradoan geosyncline in Upper Cretaceous time in North 
America, the tremendous voleanic activity of the Miocene, and the grand 
epoch of mountain-building in the Pliocene and Pleistocene. 
Professor Eliot Blackwelder touched this question in his Presidential 
Address to the Geological Section of the American Association for the 
Advancement of Science in December 1921. He concludes that there is 
no evidence of a trend towards rigidity; he sees only great pulsations 
with intermediate periods of rest. ‘The Middle Tertiary revolution was 
one of the most widespread and intense of which we have any record. 
There have been many fluctuations but no general trend. . .. The 
volcanic activity of the last two geological periods is equal in intensity 
and widespread distribution to anything known to us in earlier periods.’ 
Barrell’s observations lead to a similar conclusion ; in fact, he considers 
that the rate of sedimentation is increasing rather than diminishing, and 
gives the following general figures, the length of geological time being 
based on the minimum results obtained from uranium minerals. 
RatTE OF SEDIMENTATION IN GEOLOGICAL TIME, AFTER BARRELL. 
es Big cer Maximum thick-| time, in years 
Geological interval Time in years ness a ee a et AG 
Pleistocene ve ag 1,500,000 4,000 375 
Tertiary ie a 55,000,000 63,000 875 
Mesozoic a3 a 135,000,000 84,000 1,600 
Neopaleozoic .. “a 200,000,000 78,000 2,600 
Eopaleozoic  .. a i 160,000,000 43,000 3,700 
The last column is particularly significant from the present point of 
view ; it indicates that sedimentation was progressively slower as we pass 
backward in geological time. This observation does not mean that the 
rate of erosion of lands was slower, but that the products of erosion were 
spread over greater areas. Barrell concludes that despite the tremendous 
activity at certain times in the Precambrian, the total unrest was no 
greater than in later time, as the upheavals were separated by corre- 
spondingly long intervals of rest and erosion. The Paleozoic, also, with 
its widespread epieric seas, indicates long intervals between the periods of 
upheaval. Passing into later time, the increasing localism of deposits is 
an indication of more sustained activity—of shorter but less profound 
