[SunkenCiv]

There is geological evidence that the sunspot cycle has maintained about the same duration for many millions of years

Wow, what geological evidence would that be, tell me, I must know. :')

[hellbender]

Some sedimentary rocks contain layers which are believed to be annual (they’re called varves). Some varve sequences contain thickness variations which have a periodicity about the same as the modern sunspot cycle (i.e. roughly 11 years). Some of the variations are so regular anyone could spot them with the naked eye, like the ones I saw in some rocks very roughly 400 million years old (I’ve kicked myself ever since for not getting pictures). As someone else posted, Rhodes Fairbridge was a great expert and enthusiast for varves and other cyclic deposits.

[hellbender]

I think W. H. Bradley was one of the first geologists to recognize sunspot cycles in varved sediments. Here is a paper which confirms Bradley's idea:

http://search.datapages.com/data/doi/10.1306/D4267857-2B26-11D7-8648000102C1865D

Here's another web page discussing the varves and cycles which Bradley discovered:

http://www.indiana.edu/~ensiweb/lessons/varve.ev.pdf