When we study the geologic sections that mark the late Devonian interval in detail, we find not a single extinction boundary but several smaller ones spaced over a few million years. Several iridium-rich layers are found at this horizon. One expert, George McGhee of Rutgers, who wrote a fine book on the Late Devonian mass extinction, believes that three impacts occurred, and indeed, several craters do date to this part of geologic time.3 The Siljan crater in Sweden is the most promising; it is also of interest because it was the focus of the deep-earth methane hypothesis of Thomas Gold of Cornell University. Gold convinced himself, and then the Swedish Power Board, that the impact of a large asteroid would produce fractures that would tap deep-seated reservoirs of gases, among them methane, which then might be produced in commercial quantities. The Swedish Power Board drilled the Siljan structure but found no methane.

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