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.

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.