Possible
Worlds
that m a k e s G r e e n l a n d look so triangular on m a n y m o d e r n maps). You can understand what G o n d w a n a really looked like if y o u project f r o m the pole as centre as s h o w n on p a g e 210. T o a c o m p u t e r t h a t i s r o u t i n e . B u t w h a t e v e r t r i c k s a r e u s e d it is a l w a y s difficult to t u r n a s p h e r e into a p l a n e , a n d w o r s e still i f t h e c o n t i n e n t a l s h a p e s a r e s t r a n g e r s t o u s . C o m puter reconstructions are only as good as the information with w h i c h they are s u p p l i e d — t h e a d a g e " r u b b i s h in, r u b b i s h o u t " applies just as m u c h here as it does to dating agencies. Machines have been k n o w n to line up sad mismatches of continents, d o o m e d never to m a k e a successful marriage. In this c h a p t e r I h a v e d e s c r i b e d the w o r l d as it w a s for a few tens of millions of years during the 300-million-year history of the trilobites. It is a l m o s t a snapshot in t i m e — a time slice m i g h t be b e t t e r — b u t it is a frozen O r d o v i c i a n m o m e n t in a d y n a m i c history of a m u t a b l e w o r l d , for the continents hardly ceased in their global peregrinations. By the Silurian, 45 million years later, the o c e a n that h a d o n c e s e p a r a t e d Baltica and Avalonia from L a u r e n t i a — I a p e t u s — h a d disappeared,
subducted
away.
The
great
mountain
chain—the
Caledonides—running through the Appalachians to Scotland, and thence to the m o u n t a i n o u s fjord c o u n t r y of N o r w a y , w a s the consequence of the subsequent continental collision, an engagement almost as dramatic and complex as that which t h r e w u p t h e A l p s 250 m i l l i o n y e a r s later. T r i l o b i t e s t h a t h a d lived apart w e r e brokered into a forced marriage. T h e faunas changed in h a r m o n y with the geography. T h e eventual reopening of the Atlantic Ocean w h e n Pangaea broke up long afterwards
followed—but
only
approximately—the
same
seam as had been closed by the Caledonian orogeny in the D e v o n i a n . As a result, f r a g m e n t s of the earlier continents w e r e stranded in positions far f r o m their O r d o v i c i a n h o m e s : now, northern Scotland is on the opposite side of the Atlantic from Laurentia, to which it originally belonged—contrariwise, the two halves of Newfoundland
are welded
together today
w h e n they were originally far apart. E v e n as Iapetus h a d
209