Karen, Your story of hitting a genealogical brick wall is not uncommon. For example, the United States federal census records for 1890 were all but lost in a fire in the 1920's. This has caused me great problems trying to trace my grandfather's family through Chicago and back to Sweden.
I have heard about the Belfast fire as well. How far have you traced Erskine back in the United States? Have you found the immigration records/ships records? Those records probably exist here in the US and with those records you can begin tracing Erskine back into Ireland and you might then be able to trace Erskine from Ireland back to Scotland. It is impossible to work the other way unless you have some ancient family documents. Don't give up. There are many ways to get around the lost Belfast records to research your Erskine line back to Ireland and hopefully Scotland.