The age of those huge sequoia trees is hard to believe. What is the subject of this sentence?

What can you infer about how the particular events in "the memoirs of the conquistador bernal del diaz de castillo" still affected the writer at the time of writing his memoir? he thinks of his dead crewmen with sadness and regrets that they died for nothing. he is still frustrated that one of his men accidentally kills himself by drinking too much water.  he thinks wistfully about the wealth he may have enjoyed if they had found gold in the yucatan.  he still considers indians liars.