D A Carson on Speaking in Tongues/Glossolalia

Given that it is mentioned in the Bible, I think the issue of tongues/glossolalia will be with us until the Lord returns. I was thinking about it again recently and re-reading some of Don Carson's old book, Showing the Spirit: A Theological Exposition of 1 Corinthians 12-14 (my edition is copyright 1988, by ANZEA). So what exactly is "speaking in tongues"? Is it unintelligible, meaningless words? Is it known human language? Is it the language of angels, unintelligible to humans? Is the tongues in Acts the same as the tongues in 1 Corinthians? Here is a conclusion from Carson (taken from pp86f in my edition):

[T]here is a category of linguistic phenomenon that conveys cognitive content, may be interpreted, and seems to meet the constraints of the biblical descriptions, even though it is no known human language. Of course, this will not do for the tongues of Acts 2, where the gift consisted of known human languages; but elsewhere, the alternative is not a s simple as "human languages" or "gibberish," as many noncharismatic writers affirm. Indeed, the fact that Paul can speak of different kinds of tongues (12:10, 28) may suggest that on some occasions human languages were spoken (as in Acts 2), and in other cases not--even though in the latter eventuality the tongues were viewed as bearing cognitive content.

This is a very sensible conclusion from the Scriptural data.