Answer:
His joy for seeing the Rocky Mountains for the first time and the fear of how difficult his journey could be because of them.
Explanation:
In his journal entry from May 26, 1805, Lewis describes the "secret pleasure" he felt when he saw the majestic mountains. However, his joy was "in some measure counterbalanced" by the fear of the difficulties that crossing them could cause. He foresees the possible problems ahead, and, nevertheless, he chooses to believe it will be "a good comfortable road", meaning a successful endeavor.