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Re: islands in the Pacific-was Re: mammal mystery
>Paul Willis wrote:
> In the context of the thread, if your mountain is a true
>> granite, there is almost no chance at all of there being fossils because
>> the granite body would have solidified at a depth within the earth of
>> several miles.
>but aren't the Rockies an example of granite mountains that were formed
>ABOVE ground during the Cretaceous? And wouldn't a fissure-fill be
>quite likely to hold Cretaceous fossils in the Rockies, since there is
>every liklyhood that there was dinosaur fauna in the area?
Only a small part of the Rockies consist of granites which can be broken
into two types; those that formed before the Rockies appeared and were
subsequently exposed when the mountains were upliftred and eroded and those
that formed deep within the Rockies early in their history (around the late
Cretaceous) and have been subsequently exposed by uplift and erosion. As
stated before, the true granites formed deep with mountain chains and
rarely erupt through to the surface. remember, even if granitic magma does
breack the surface, it will form rhyolites, not granites.
Any granites actually forming when the dinosaurs walked the earth would
have been a) a few kilometres under the surface and b) many thousands of
degree in temperature. This is an impossible environment for fossilisation.
Fissure development withing granites would probably only work where a
grnaite is dissected by a later magma dyke that weathers at a faster rate
than the granite. This, when the rocks are exposed to weathering, the dyke
could erode into a fissue that could subsequently refill. However, such
fissure-fill structures are rare and acidic, a bad environment for
fossilising bone. Off the top of my head, I cannot think of a single
example of a vertebrate fissure-fill deposite in granite.
Dr Paul M.A. Willis
Consulting Vertebrate Palaeontologist
Quinkana Pty Ltd