I agree that the antecedent of 'which' is 'a cause'.I have a trouble with the antecedent of the sentence below. Is the antecedent of WHICH "a cause?" Is there any possibility WHICH expresses "a cuase and a natural law?"
>Every movement in the market is the result of a natural law and of a cause which exists long before the effect takes place and can be determined years in advance
The main reason is that 'exists' is singular, and if the antecedent were 'a natural law and of a cause', plural 'exist' would be called for.
Secondly "cause and effect" is a common English expression, and they are probably being matched up together in this sentence without 'a natural law'.