The base-word of each of these expressions is a noun, called an adverbial noun. It is immaterial to us what case this noun is in, but a study of Old English teaches us that it is in the objective case, while a further proof is that we frequently employ a preposition before it.
The adverbial noun usually denotes measure of some sort, of time, distance, space, weight, value, etc.
What the Adverbial Noun modifies.—1. A verb; as, “A man could have found places where he could have jumped three thousand feet in one descent to the valley.”—King.
2. An adjective; as, “He was nearly a head taller than myself.”
The adjective worth is always modified by an adverbial noun answering the question how much? and frequently by others answering various other questions; as, “Wheat worth a dollar a bushel last week is selling now for eighty cents.” Notice that the modifiers of worth differ in closeness of relation; the closest is dollar, the next bushel, and the next week. This is not determined by position.
3. An adverb; as, “The oldest sergeant present stepped a pace forward.”
4. A phrase; as, “Centuries before this the Chinese had printed books by means of carved wooden blocks.”
5. A clause; as, “His room is hung round with cases of them, each impaled on a pin driven through him something as they used to bury suicides.”—Holmes.
In such a sentence as, “He is forty years of age,” it is somewhat difficult to determine whether years modifies of age or of age modifies years. This arises perhaps from the fact that neither years nor of age makes a good base for the complement of is. He is not forty years, neither does the sentence mean that he is of age. We must say that is is completed by the expression forty years of age, used adjectively and having the meaning of one word. If we wish to go further and analyze this complement, we say that of age is the base, meaning old, and that this phrase is modified by the adverbial noun forty years.
It is the same in the sentence, “We often read about precipices that are thousands of feet in descent.”—King. The verb are is completed by the group of words, thousands of feet in descent, having the meaning of very high. Of this group in descent is the base modified by the adverbial noun thousands of feet.