No mention of rock anywhere in the contract!
No there wouldn't be.
Hitting rock is an unforeseen, it is not an extra, it is not a change.
I was once very well up on the standard construction contract used by the public sector, and for many years it put the cost of unforeseen items on the employer (the customer in effect). The logic being that if there is rock the builder has more work to do and so is entitled to charge extra.
Then the standard contract was changed to put the cost of such items on the builder, the logic being that builder was the expert and should have done his homework before giving a price.
I have no idea what your contract says about unforeseen items, or if there is any disclaimer built into the builders quote. If the contract is silent I would think that you are in a strong position to argue that the builder should pick up the tab, for two reasons.
1. He should have done a proper investigation to find out what work was needed before he gave you a price.
2. The standard contract in the public sector puts this cost on the builder. Obviously this is not a public sector job, but that standard contract accounts for the majority of work in Ireland and could be looked to as standard practice.
Question, did you employ an engineer or an architect ?