I was reading Harry Potter and the Philosopher's Stone and then I happened to notice something like this:
The mountains around the school became icy grey and the lake like chilled steel.
I understood it as "the lake was like chilled steel" though the auxiliary verb had been omitted. I remembered catching several other occasions of this. One example is:
'I'll be back at dawn,' said Filch, 'for what's left of them,' he added nastily, and he turned and started back towards the castle, his lamp bobbing away in the darkness.
The auxiliary verb seemed to be missed intentionally in the last phrase of the sentence, as I read it as "his lamp was bobbing away in the darkness."
I just wonder if this is grammatically correct? Is there any difference between having the auxiliary verb and omitting it in these cases? Is there any special meaning by omitting the auxiliary verb in these scenarios?