228 
of the saltness of the sea, as it was about 
2,000 years ago; for then”, he con- 
tinued, “it cannot be doubted but 
that the difference between what is 
now found and what was then, would 
become very sensible.”” Now cb- 
viously, if Halley had been thinking 
in millions of years instead of thou- 
sands, he must have realized that the 
increase of salinity since Roman times 
would be quite undetectable. Never- 
theless, he did not fail to slip in his 
suspicion that “‘the world may be 
found much older than many have 
hitherto imagined.” 
It was James Hutton (1726-1797) 
who first clearly grasped the full 
significance and immensity of geolog- 
ical time. In his famous ‘“Theory 
of the Earth,’? communicated to the 
Royal Society of Edinburgh in 1785, 
he presented an irrefutable body of 
evidence to prove that the hills and 
mountains of the present day, so far 
from being everlasting, have them- 
selves been carved and are still being 
modified by slow but inexorable proc- 
esses of erosion such as those now in 
operation; and that the sand and mud 
continually removed by rivers are 
being slowly deposited on the sea floor 
as sedimentary rocks in the making. 
Realizing that “‘the past history of our 
globe must be explained by what can 
be seen to be happening now,” and 
observing that the sedimentary strata 
of the earth’s crust bear all the hall- 
marks of having accumulated exactly 
like those now being deposited, he 
saw that the vast thicknesses of these 
older strata implied the operation of 
erosion and sedimentation throughout 
a period that could only be described 
as inconceivably long. But Hutton 
went further. He recognized not only 
that the earth is a thermally and dy- 
namically active planet, internally 
as well as externally; he was also the 
first to demonstrate that the internal 
activity is of a cyclic character. He 
saw that the present cycle of erosion— 
given only time enough—would even- 
tually reduce the most vigorous land- 
scape to sea level, and he deduced 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
from the very existence of landscapes, 
carved for the most part out of marine 
sediments, that these sediments must 
have been upheaved from the sea floor 
in some former age. In unconformi- 
ties, such as the classic example illus- 
trated by plate 1, figure 1, he found 
the “‘ruins of an earlier world”? which 
had already passed through a similar 
cycle of upheaval and erosion long 
before the present one began. The 
story revealed by the structure and 
sequence of the rocks at Siccar Point 
is clear. The lower rocks, now stand- 
ing nearly vertical, are sediments, 
which, originally deposited on the sea 
floor, were subsequently folded by 
powerful lateral compression. The 
region of folded and therefore greatly 
thickened rocks then rose to form a 
land mass which was attacked by 
denudation as soon as it appeared 
above sea level, to be gradually dis- 
sected into mountains and valleys and 
eventually reduced to a plain. As a 
result of later subsidence, the worn- 
down surface became an area of de- 
position, and successive layers of 
sediment then slowly accumulated 
over the truncated edges of the older 
rocks. The plane of unconformity 
separating the up-tilted rocks beneath 
from the flat-lying rocks above evi- 
dently represents an immensely long 
interval of time. 
Today we know that the earth’s his- 
tory has included a succession of at 
least 10 of these major cycles, each 
involving (1) thick accumulations of 
sediments and volcanic rocks in a 
subsiding belt of the crust; (2) intense 
compression of the belt, resulting in 
folding and crumpling, and accom- 
panied by metamorphism of the deeper 
rocks and the formation of great masses 
of granite; and (3) general uplift of the 
belt and the wearing away of its ex- 
posed portions by denudation. The 
Alps and Himalayas are examples of 
mountain systems now in stage (3) of 
the latest of these mountain-building 
or orogenic cycles, as they are called. 
The rocks of Cornwall and Devon are 
relics of the immediately preceding 
