In my Foreword to The Peoples of Middle-earth, pp.ix-x, I referred to the forerunner of the History [of Middle-earth] as 'an entirely "private" study, without thought or purpose of publication: an exhaustive investigation and analysis of all the materials concerned with what came to be called the Elder Days, from the earliest beginnings, omitting no detail of name-form or textual variation.' This work, which I called The History of the Silmarillion, and which I began after the publication of my 'constructed' text, runs to more than 2600 very closely typed pages, and it does not even touch on the Second and Third Ages. When the possibility arose of publishing at least part of this work, in some form, it was obvious that it would have to be heavily reduced and curtailed, and the part of The History of Middle-earth dealing with the Elder Days is indeed a new presentation of The History of the Silmarillion, and a severe contraction of it, especially in respect of the sheer quantity of variant manuscript material reproduced in full.

-Christopher Tolkien