A British geologist and historian was in Bermuda this month to take a crack at solving a riddle that confounded famed scientist Charles Darwin: Bermuda's coral reefs.

Brian Rosen, a scientific associate at the National History Museum in London, gave a lecture at the Bermuda Institute for Ocean Sciences (BIOS). The lecture was entitled 'Red or Blue? Darwin's Bermuda Dilemma and his Enduring Subsidence Theory of Coral Reefs'.

Dr Rosen is married to Bermudian Jill Darrell Rosen, and is the son-in-law of Bermudians Pam and Owen Darrell. He has visited the Island many times with his wife.

"I have been interested in the history of coral reef studies for a long time," said Dr Rosen. "Last year was Darwin's bicentennial and the sesquicentennary of publication of his book 'The Origin of Species'. He is one of the heroes of coral reef studies."

Although Darwin is best known for his work on evolution, he was also a geologist. His first three scientific books, written between 1842 and 1846, explored the geology of coral reefs, volcanic islands and South America respectively, and gained him great recognition in this field.

"Darwin's subsidence theory is impressively elegant and far-sighted in terms of the origin of atolls," said Dr Rosen.

An atoll is an island consisting of a circular coral reef surrounding a lagoon.

"He was particularly interested in global patterns and processes; his subsidence theory was based on the idea that the world's ocean floors as a whole were subsiding, and that atolls provided the surface evidence of this process, having grown on sinking foundations of former subaerial volcanoes," said Dr Rosen.

"The sheer scale and simplicity of this proposition was reflected in his famous coral reef map, on which he shaded atolls and most other oceanic reefs in blue [inferred subsidence], and most reefs elsewhere in red [inferred uplift or stability]."

Darwin's coral reef theory was widely-acclaimed initially, but later seemed too audacious for many, and became heavily disputed from the latter part of the 19th century onwards. One problem was that the theory did not take into account the advance and retreat of polar ice caps during the various ice ages.

But geologic drilling of bore holes in the 1950s vindicated him and showed that the reef deposits of most atolls did indeed extend hundreds of metres beneath their surfaces far deeper than can be explained by glacial sea level changes alone.

In 1831, Darwin set sail from Plymouth Sound, Plymouth, England on a five-year journey on the HMS Beagle around the world. He spent part of the journey studying coral reefs. He never visited Bermuda, but he did correspond with a Lieutenant Nelson about Bermuda's corals.

"Nelson did a very nice geological account," said Dr Rosen. "It was very early for such things anywhere. It is very interesting. I was reading it at the BIOS library just a few days ago."

But Nelson's observations confused Darwin, because they didn't fit entirely with his theories of coral reef formation.

"Some of the things that Nelson told him were puzzling," said Dr Rosen. "I thought coming here I could combine my interest with Darwin's study of coral reefs with this particular problem about Bermuda. He couldn't quite fit it in."

So while Darwin drew up a map and coloured various coral reef formations and atolls red or blue, he didn't colour Bermuda at all, because he was undecided.

"We have an atoll shape, but he was troubled with the fact that it didn't seem to have subsided," Dr Rosen said. "There were details of the Island's geology that didn't fit with subsidence. I think that was holding him up. In fact, you can accommodate it."

He said scientists now know that the patterns of subsistence on the ocean floor are caused by plate shifting.

"Starting at the Mid-Atlantic Ridge and moving away, on either side the ocean floor gradually drops. It is not much but it is enough to give you subsidence on an atoll."

The problem is that the Bermuda seamount does not seem to be subsiding very much at all, despite the fact that it is about 45 million years old.

"For that age it should have subsided more," said Dr Rosen. "And this is a problem that is not fully resolved. The seamount is not really doing its standard things. All the bore holes done here show that the volcanic material is quite shallow. It is 30 to 100-plus metres down. That base is the same age but the volcano is 12,000 metres down. It isn't sinking very much."

Residents will be relieved to know that the Bermuda seamount is so stable that it can be used as a gauge for changes in sea level.

"Scientists say, let's look at Bermuda to measure any changes in sea level, because the Island itself is not going up or down by very much," said Dr Rosen.

In fact, Dr Rosen said changing sea levels probably contributed to Darwin's confusion.

"At that time, he would not have understood that sea levels are changing all the time. But you can see evidence of this by visiting places like Rocky Bay to the east of Devonshire Bay. You can see fossils locked into the rock which show the changing sea levels, and you can see where the current beach is. It is a fascinating little place."

Dr Rosen believed that if Darwin had visited Bermuda, he might have cottoned on to the fact that sea levels have fluctuated over time.

"He was an extremely competent geologist," said Dr Rosen. "I think he would have realised that the sea levels must have changed. I think he would have twigged it. His notes show debates with himself, and that he was thinking alternative ideas. I think he would have started to wonder if sea levels were changing."