Final answer:
The author's thesis in the passage is that the Grand Canyon's beauty cannot be adequately represented using language or visual arts.
Step-by-step explanation:
The two sentences that best state the author's thesis in the passage are:
The wonders of the Grand Canyon cannot be adequately represented in symbols of speech, nor by speech itself.
Language and illustration combined must fail.
These sentences convey the author's argument that the immense beauty and awe-inspiring elements of the Grand Canyon cannot be fully captured or described using language or visual arts. They suggest that the Grand Canyon is a phenomenon that exceeds human comprehension and defies representation.