a miniature cavern

That's a tough one. Can you provide more context?

Yes, I should have provided the context:
'Ah, so's -- here you be! -- Ah, Michael and Joseph and John -- and you too, Leaf! a merry Christmas all! We shall have a rare log-wood fire directly, Reub, to reckon by the toughness of the Job I had in cleaving 'em.' As [William] spoke he threw down an armful of logs which fell in the chimney-corner with a rumble, and looked at them with something of the admiring enmity he would have bestowed on living people who had been very obstinate in holding their own. 'Come in, grandfather James.'

In this story, people are around the fire-place, and thus, William 'was discering' 'the mouth of a miniature cavern' in the fire.

I think that with a little effort I could imagine a miniature cavern in the dark areas of a fire:

Thank you so much for your help, FBM.

No prob. ^^

I'm currently translating Hardy's Under the Greenwood Tree into my language. I truly appreciate your help.

What is your native language?

Japanese -- it's so difficult to render Dorset dialect into Japanese!

I can imagine. I once tried doing something like that from English to Korean, but gave up. The nuances are too numerous and too subtle. The reader wouldn't understand without copious footnotes, anyway, and those notes would be longer than the original text. A real quagmire.