Final answer:
The phrase 'castles in the air' is an example of a symbol in the excerpt. It represents the protagonist's unrealistic dreams about marrying his cousin.
Step-by-step explanation:
The phrase 'castles in the air' is an idiom used in the excerpt from Journey to the Center of the Earth. It is an example of a symbol. In this context, it represents the protagonist's dreams and fantasies about marrying his cousin. The phrase suggests that these dreams are unrealistic and not likely to come true.
Learn more about Symbols in literature