Hi and welcome to Just Answer!
Both selling transactions - the surface rights and the mineral rights - are treated as he sale of partial rights related to the ownership - so it is the sale of a real property - and you are correct - the gain is treated as long term capital gain and is taxed at reduced rate - not more than 15%.
Your taxable gain = (selling price) - (basis).
However - if there is a large long term capital gain - you need to verify if you will be subject of AMT tax.
The basis of an inherited property is a fair market value of that property at the time the decedent died.
As you need to prorate that basis between the surface rights and the mineral rights - if selling prices are about the same - I would prorate the basis in the same proportion.
If you have a ground for a different proration - you may prorate the basis differently. As long as the proration would be reasonable - it will be accepted by the IRS. You may also hire an appraiser to do historical appraisal - but might want to consider the service cost.
The sale transaction is reported on form 8949 - Part II - http://www.irs.gov/pub/irs-dft/f8949--dft.pdf - if the property was held more than a year - box "C" in the header should be checked.
Then - amounts will be transfered to schedule D - http://www.irs.gov/pub/irs-pdf/f1040sd.pdf
Part II, line 10 - and you should have the taxable gain in column (h).
Let me know if you need any clarification this matter or have other tax related issues.