[Legate of Amon Lanc]

Ah yes, you are right, I forgot. Thanks for the reminder. However, I am not sure about the Ered Engrin thing. It's from HoME I suppose (that thing about Yellow Mountains)? I'm not sure, but I think that the symetry of Valar was more west - mirror - east, not north - mirror - south. I suppose also the White Mountains (more or less, going from west to east, thus, if they were a part of the "original" creation of Arda, they'd have to had a counterpart on the south) were created by some natural (well, "natural"...) disaster.
I also doubt about the Yellow Mountains (I'd be grateful if you find any information about them that you post it here) being mirror to the Iron Mountains, because:

Originally Posted by Silmarillion 14

In the north of the world Melkor had in the ages past reared Ered Engrin, the Iron Mountains, as a fence to his citadel of Utumno.

It says explicitely that Ered Engrin were reared by Melkor - so they couldn't be the part of the original "plan" of Valar, thus, not being symetrical with any of the other mountains.
However, it is also said in the first chapter of Silmarillion, that

Now Melkor began the delving and building of a vast fortress, deep under Earth, beneath dark mountains where the beams of Illuin were cold and dim. That stronghold was named Utumno.

Which seems that the mountains already were there. Not that Melkor couldn't have first reared the mountains and then started delving and building Utumno - but we are told in the previous quote, that he reared them as a fence, thus, I'd think that he did this after he had something to make a fence around, right?
Perhaps in the first quote the importance is not put on that Morgoth reared the mountains (meaning he "built them"), but to that he reared them around Udun. Ergo, the mountains were there (created by Valar), but he somehow worked on them and changed their ranges, thus, they now created a fence around his new citadel. (I know, a poor assumption. I'd rather stay with the first one - see below.)

One more thing - concerning Hithaeglir and the height of the mountains, as I implied in my previous post.
I wonder if the Hithaeglir was the highest of the mountains in ME? We are not told as much about the other mountain ranges as about the Misty Mountains, from what is described in the Hobbit and LotR, Hithaeglir seems pretty high. And also, we have the testimony of the
Silmarillion 3 - Of the Coming of Elves and the captivity of Melkor:

...and beyond it were mountains whose sharp horns seemed to pierce the stars. (...) But the mountains were Hithaeglir, the Towers of Mist upon the borders of Eriador; yet they were taller and more terrible in those days, and were reared by Melkor to hinder the riding of Oromë. (...) And when Oromë was gone forward the Teleri looked upon the shadowy heights and were afraid.

If we take into consideration that the Hithaeglir was raised by Melkor, then we can assume that it might have been as high as the Ered Engrin (or Ered Engrin could be as high as Hithaeglir). When we are told that they were "taller and more terrible in those days", and what we know from LotR, the Misty Mountains were pretty tall. The main thing I'd assume is that Hithaeglir was much more higher than Orocarni. Why: the Elves have possibly seen Orocarni at their awakening near Cuiviénen, so if Orocarni were as tall or even taller than Hithaeglir, they probably wouldn't be so panicked. Although they probably didn't wander the Orocarni, you are not so awed of something you know. Thus, I'd assume that the elves first saw mountains of that height, so Hithaeglir must have been far higher than Orocarni, which mean also higher than Ered Luin (symetry). Or different way, in the same chapter where we are told about the horror of the elves when they saw the Misty Mountains, there is later laconically written that "Vanyar and Noldor came over Ered Luin, the Blue Mountains", and there is no problem with it. Also I am quite convinced that the Ered Nimrais as well as the Mountains of Mordor were smaller.
What is the most interesting part in my opinion is however the following. Since it is possible that the Grey Mountains were a part of the Iron Mountains of Melkor's (see my post above) and they meet with Hithaeglir near Mount Gundabad, I find it very likely that these mountains (Hithaeglir&Ered Engrin) were actually of the same height! As I implied before, there was no
And one more thing. Maybe you noticed in the quoted text from the 3rd chapter of Silmarillion one connection of words which seems like if we have seen it somewhere before. We are told that the Misty Mountains "were taller and more terrible in those days, and were reared by Melkor to hinder the riding of Oromë".
The same words like those of Ered Engrin, that they were reared by Melkor. If the words have the same meaning here, it would mean that Ered Engrin were really built by Melkor (because of Hithaeglir we can tell for sure that it was not built by the Valar, and if Ered Engrin were "made" the same way...). Thus, this would mean that Ered Engrin wouldn't fit into that symetry of Arda, they were completely built by Melkor.