Genesis 1 & 2 and the Perspicuity of Scripture

A few days ago I read something by the esteemed theologian and pastor, G.I. Williamson, that both surprised and disappointed me.  His comment was made concerning the interpretation of the first two chapters of Genesis. 
"The word ‘perspicuous’ here means clear—clear enough for you and I to understand it."

In short, according to Williamson, there is only correct interpretation of Genesis 1 & 2 is the literal six-24 hour day view.   

I realize that the word, "perspicuous" and its derivative, "perspicuity" are not part of everyday English.  What do they mean?  How do they apply to Genesis 1 & 2?

In Reformed and Presbyterian circles, these terms have been applied to Scripture concerning salvation not creation.  This is where Dr. Williamson gets off track.  The Westminster Confession of Faith 1:7 puts it this way:  

"All things in Scripture are not alike plain in themselves, nor alike clear unto all: yet those things which are necessary to be known, believed, and observed for salvation (my emphasis), are so clearly propounded, and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them."

Thus, the perspicuity of Scripture has been understood to mean that the Scriptures are plain enough concerning salvation that even an unlearned person, led by the Holy Spirit, can understand what God requires for salvation. 
The great reformed theologian Louis Berkhof makes much the same point.  Contrasting the Roman Catholic position that the Bible is obscure and nearly unfathomable Berkhof writes this about the Protestant Reformers:

"Over against this position of the Roman Catholic Church the Refomers stressed the perspicuity of Scripture.  They did not intend to deny that there are mysteries in the Bible which transcend human reason, but freely admitted this.  Neither did they claim such clarity for Scripture that the interpreter can well dispense with scientific exegesis.  As a matter of fact, they engaged in exegetical labors far more than the votaries of Rome... Their contention was simply that the knowledge necessary unto salvation, though not equally clear on every page of Scripture, is yet conveyed to man throughout the Bible in such a simple and comprehensible form that one who is earnestly seeking salvation can, under the guidance of the Holy Spirit, by reading and studying the Bible, easily obtain for himself the necessary knowledge, and does not need the aid and guidance of the Church and of a separate priesthood."  Source: Louis Berkhof, Systematic Theology New Combined Edition, p. 167 of the Introductory Volume.

For those wishing to delve deeper, a better article explaining perspicuity and the problem with Williamson's line of reasoning can be found here.