Adam Clarke on Amos 6:12

Shall horses run upon the rock - First, they could not do it, because they were unshod; for the shoeing of horses with iron was not then known. Secondly, If they did run on the rock, it would be useless to their owner, and hurtful to themselves. Thirdly, And it would be as useless to plough on the rock with oxen; for there it would be impossible to sow with any advantage. Fourthly, Just as useless and injurious would it be to put gall in the place of judgment, and hemlock in the place of righteousness. You have not only been laboring in vain for yourselves, but you have also been oppressive to others; and for both ye shall suffer.