[ Pobierz całość w formacie PDF ]

speculation, we need some corroborating evidence. The problem is similar to
that of deciding if the two doors of a closet have been cut from the same
piece of timber, in which case the obvious thing to look at first is the grain
pattern across the join. It's important, of course, to distinguish between
similarities that mean something and others that don't, such as scratches made
after the doors were hung in place. In the case of continents, an example of
the latter would be the evidence of extensive rain forests which geologists a
few million years from now will observe in the basins of the Amazon and the
Congo. Obviously they would be wrong if they inferred from this that the two
places had been joined in the twentieth century the match would be simply the
result of similar environments working on similar raw materials.
The true "grain" of continents consists of the mountain belts and old, deep-
lying rock strata. When these significant geological formations were
distinguished and compared for age and structural characteristics, they showed
an extraordinarily good correspondence across the best-fit join selected by
the computer analysis. Furthermore, nothing comparable to them appears in the
structure of the ocean floor separating the two regions today. Now if somebody
observed that the two closet doors matched but the fixed upright strip between
them was different, he would conclude that the doors had been cut from the
same piece of wood at some time, and the upright inserted between them
afterward it's the simplest explanation that fits the facts. The same
conclusion followed, too, for the continents. Data on the ages of the relevant
rock formations indicated that the join had persisted until at least five
hundred million years ago. All this was not especially new. As long ago as
1912, the German meteorologist Alfred Wegener based his then revolutionary
proposal of continental drift on exactly this kind of information, and his
work was subsequently expanded by many investigators.
So, we have five hundred million years as an upper limit for the date of
separation. How about a lower limit? It turned out that the areas of overlap
in the best-fit solution enable a date to be fixed for this, too. Any parts of
the continents that overlap must have formed after there was a gap for them to
form in. One of these regions was the Niger Delta. The sediments that make up
this formation are all younger than fifty million years and extend well over a
hundred miles into the Gulf of Guinea. This says that the separation must have
been at least this much by that time. To narrow these limits down further, we
need to introduce glaciation and ice sheets into our story.
Between 350 and 250 million years ago, the continents of the southern
hemisphere were covered extensively by ice. Rocks can be carried over enormous
distances by moving ice, and are dumped wherever they happen to have got to
when the ice melts for example, Norwegian rocks carried by the glaciers of the
most recent ice age are quite common in parts of Britain. Huge glacial
deposits exist today all over eastern Brazil, which appear to have resulted
Page 132
ABC Amber Palm Converter, http://www.processtext.com/abcpalm.html
not from the melting of glaciers localized in valleys, but of vast sheets of
ice. In some places these deposits are more than two thousand feet thick about
ten times the depth of the deposits left by the recent ice age in Europe. The
thrust patterns in the wrinkles and folds of the underlying rock indicate that
the ice moved from southeast to northwest, i.e., from somewhere in the
direction of the Atlantic. Well, where did all that material come from?
Let's be scientific and consider the alternatives before jumping to
conclusions. Is it possible, for instance, that a large landmass once existed
between today's coast and the continental shelf, which was shoveled up by the
ice and carried inland? Not really. All the studies that have been made of
glaciation indicate that ice sheets don't work that way. They scratch and
polish existing terrain, and carry away the looser debris, but they don't
grind whole slabs of continent down to nothing. And besides, although one
hundred million years sounds a long time, it isn't anywhere near long enough
for that kind of major surgery. Very well, could there have been another
continent offshore in what today is the South Atlantic an "Australantis?" No.
This conjecture runs into trouble, too, for despite romantic legends to the
contrary, continents don't sink beneath the sea. Oceanic crust the material
that forms the floors of the ocean basins is entirely different from
continental crust. The floor of the South Atlantic is perfectly normal, which
means that any continent that once existed there would somehow have had to
transform itself from twenty-five-mile-thick continental crust into five miles
of oceanic crust plus twenty miles of upper mantle (the deeper layer that lies
beneath the crust all over the earth), or else have disappeared without trace.
So let's take the simple way out again, and go back to our original idea. If
Africa was joined to Brazil at one time, we have a ready-made source for all
those Brazilian glacial deposits. What's more, investigations in western
Africa revealed widespread evidence of glacial erosion in an east-west
direction i.e., out into the Atlantic but very little in the way of subsequent
deposits left by melting. And as a clincher, the Brazilian deposits include
many erratic blocks of such rock as quartzite, dolomite, and chert, which
resemble none of the structures that make up Brazil, but which are common in
southwest Africa. Quantitative analysis of the glacial evidence brings the
upper limit for the date of separation down from five hundred million years to
two hundred million. We still have fifty million years as the lower limit.
During the period that lasted from 135 million to 100 million years ago (Lower
Cretaceous), the strips which today form the South American and African
coastal regions both consisted of chains of sedimentary basins low-lying
flooded areas where successive layers of rocks were laid down, the types
differing as depth and other environmental factors changed. The sequences of
the sediments found on both sides of today's ocean are similar, and bear no
resemblance to the basin floor between the continental slopes. And this, of
course, is just what would be predicted if both sequences were in fact formed
as parts of the same process at a time when the intervening basin didn't
exist. The case seems to be getting stronger. For a better idea of what kind
of process this was, we need to turn to biology.
Fossil remains of fish and other organisms preserved in these sediments,
particularly the lower layers, include many freshwater species that could
never have survived in seawater. This implies that at up to about the same
time on both edges of what is today the Atlantic, the water that was laying
down the sediments was not ocean. Then, above the freshwater deposits, we find [ Pobierz całość w formacie PDF ]