Non-standard "because" usage - is it valid?

I've currently reading Neal Stephenson's book "The System of the World", and there have been multiple instances of him using a sentence structure with "because" that seems strange.

As an example, one sentence of the form in question is:

Jack was silent for a while, because alert.

I would normally expect to see this written something like:

Jack was silent for a while, because he was alert.

That is, he seems to omit repeating the subject. Is this structure valid? I read quite a bit, but can't recall ever seeing anyone else do this.

Editing to add another example of this structure, which I believe is now the third or fourth time I've seen it in this book:

Upon entering the Chapel, every denizen of Newgate stops in his tracks for a few moments because staggered by a blast of light, a sort of optical fanfare.

Solution 1:

It is grammatically incorrect, because the clause has no subject. Of course it is not essential to always be strictly grammatically correct. People often violate the rules for effect. The problem is when someone violates the rules for no apparent reason.

In this case I don't see how it achieves any useful effect, but I'm just looking at one sentence out of context.

I can't think of any other examples of this sort of construction off the top of my head, other than from people who where obviously struggling with the language.

Solution 2:

This is an example of ellipsis, the omission of a word or words from a sentence. We use it all the time in conversation. There is nothing ungrammatical in your example, but the extent to which it might be effective in a novel (which I take ‘The System of the World’ to be) more properly falls under the heading of literary criticism.