highland-piper, that simply isn't true. Just because many tunes were published during that period in collections such as the Atholl, Skye, Gesto, Gunn etc does not mean the tunes were from that period. Many tunes in these collections are listed as 'very old'. The Scottish tune repertoire is actually enormous, much more so than the Irish for example and while it's true to say that it's well documented that many tunes were written in that period it's not true that half of all Scottish tunes were written in that period, or anything like it.
I said that quite a few that are currently played are modern, and this is certainly true. Many of the composers are still living.
The balance were therefore written previously, and it is established fact that they were mostly written in the mid 1700s to early 1800s (aside from the prodigious Scott Skinner). We know who wrote most of the good tunes and when. Sure there are tunes of unknown origin, even some good tunes, but not nearly as many as those with known provenance. Edinburgh was a (if not the) center of enlightenment, so it is no surprise that the men who wrote the popular music of the day were well known (not to mention generally highly educated).
If a tune was published in say 1780, then we know it was not composed at a later date. We know that the music we have today took form after the introduction of the Italian violin to Scotland, perhaps in the last few years of the 1600s. Because Scottish publishers were so prolific (cultural center and all), it is incomprehensible that there would be good tunes unpublished for any length of time. If you subtract out everything played today that wasn't written by a known composer there's not a lot left. It is interesting that in many cases later publications omit composer names that were printed in earlier publications, perhaps lending to the impression that they are of unknown origin.
See David Johnson's "Scottish Fiddle Music in the 18th Century" for a scholarly, yet entertainingly readable treatment of the subject. Also his ’Music and Society in Lowland Scotland in the 18th Century’’