At the time of Federation, the colonies' main source of revenue consisted of customs and excise duties (income tax being still a newer notion). Since one of the main reasons for Federation was to create a common market, inevitably authority over these taxes was vested exclusively in the Commonwealth Parliament (section 90). It was acknowledged that this would create a situation where the Commonwealth would raise much more money than it could spend, whereas the States, being still responsible for most areas of law and of social infrastructure, would need to spend much more money than they could raise (the problem now known as "vertical fiscal imbalance"). Although the framers were able to agree on a formula for distribution of the Commonwealth's surplus to the States in the first few years after Federation, they could not agree on a long-term formula. Accordingly, section 96 of the Constitution provides that the Commonwealth Parliament "may grant financial assistance to any State on such terms and conditions as it thinks fit".