obobob Wrote:I was sort of thrown off when I read the first three sentences. So are the sentences from the third sentence (“First, scientists noticed that the ocean floor exhibited ...”) to the rest of the paragraph going against the old belief in the first sentence?
Yes, that's essentially what's going on.
The passage starts contradicting the old belief right away, right in the second sentence ("But this idea became unsupportable..."). The third sentence starts to describe the new discoveries that made the old idea unsupportable. "First" indicates that the discovery of the odd magnetic variations was the first of the new discoveries mentioned in the previous sentence.
The rest of the first paragraph provides more information about these magnetic variations. The rest of the first paragraph—and really, the rest of the passage—builds off of these first three sentences.