Here ends the Silmarillion.

[Galorme]

If it has passed from the high and the beautiful to darkness and ruin, that was of old the fate of Arda Marred; and if any change shall come and the Marring be amended, Manwe and Varda may know; but they have not revealed it, and it is not declared in the dooms of Mandos.

There are really two Marrings of Arda. One was the physical corruption of the earth, the turning of Yavanna's works to evil, the bending of the earth, the destruction of the symmetry, and the corrupting of the elves (or whatever Melkor corrupted to make the orcs). The other was done before the start of time, during the Ainurlindalë, where Melkor put his own ideas into the music. Which do you think was the most damaging?

Also that statement seems to suggest something deeper. Like the natural tendency for things to decay was caused by Morgoth. Maybe Melkor introduced the Law of Entropy into the Music? It seems that the idea of lasting beauty, or things getting better with time never occurred to JRRT. To be honest I think I pity him that.

[HerenIstarion]

One was the consequence of another

Your pity is, fortunately, vain:

Never since have the Ainur made any music like to this music, though it has been said that a greater still shall be made before Ilúvatar by the choirs of the Ainur and the Children of Ilúvatar after the end of days. Then the themes of Ilúvatar shall be played aright, and take Being in the moment of their utterance, for all shall then understand fully his intent in their part, and each shall know the comprehension of each, and Ilúvatar shall give to their thoughts the secret fire, being well pleased.

[Amarie]

I agree with the examples you give of the "first" marring..

One was the physical corruption of the earth, the turning of Yavanna's works to evil, the bending of the earth, the destruction of the symmetry, and the corrupting of the elves (or whatever Melkor corrupted to make the orcs).

I do not agree with this second though...

The other was done before the start of time, during the Ainurlindalë, where Melkor put his own ideas into the music.

I see the music as an event which Iluvatar used to mature the Ainur, to bond them together,let them learn to think and further devople who they were. So I do not beleive that Melkor's dark thoughts that happend during the music was a marring of Arda, but a marring of himself. It was a shaping of Melkor into someone who would seek to destroy all that was "good".Tthose are just my ideas though and they could be wrong, even directly contradicted by something factual without me realizing it.
Galorme, when you say...

It seems that the idea of lasting beauty, or things getting better with time never occurred to JRRT. To be honest I think I pity him that.

Things getting better with time is a thing that was included in his writings. As for the lasting beauty, I see nothing wrong in all things coming to an end in his writing. Nothing that we as humans can comprehend will certainly last forever. The book does not end without hope. We do not find out what becomes of men when they leave this world, so that is not a certain end. Also, in The Lord of the Rings there is talk about hope for the future.
As for this...

Also that statement seems to suggest something deeper. Like the natural tendency for things to decay was caused by Morgoth.

I believe so. Without Melkor's destruction Middle-earth may have been like Valinor.
I also believe that it was the will of Iluvatar that this be, so the marring is not (in my opinion/observation) entirely Melkor's fault, just as the creation of Arda was not entrely the fault of the Valar...but also Iluvatar himself.

[ October 22, 2002: Message edited by: Amarie ]