I just discovered a gap in my understanding of restrictive vs. non-restrictive clauses that I hope somebody can fill.  Here is the text that created the problem for me:



		Early astronomers, who considered Mars to be the best candidate for extraterrestrial life, thought they saw straight lines crisscrossing the planet.  They thought that the straight lines were irrigation canals that had to have been built by intelligent beings.



Clearly, the relative clause in the first sentence is non-restrictive.  However, the relative clause in the second sentence is also non-restrictive in that it does not help the reader to restrict or identify which irrigation canals the writer is discussing.  And yet my internal grammar clearly marks this one as being restrictive.  What am I missing??



Thanks,



John