Which parts of this excerpt from ozymandias by percy shelley reflect the theme that art alone can last forever

The poem is a sonnet so it's not that long. Most part of it shows that art lives on but maybe the most prominent part is 
Tell that its sculptor well those passions read Which yet survive, stamped on these lifeless things, 

because it points out that art keeps surviving and before and after it it describes the barren wasteland around it. You did not provide the excerpt but this would be a close answer.

It would be