I was wondering if someone here would be able to clarify something for me.
I'm currently analysing a sentence from 'Hound of the Baskervilles', and I've come across this:
"Mr. Sherlock Holmes, who was usually very late in the mornings, save upon those not infrequent occasions when he was up all night, was seated at the breakfast table."
My first thought was to regard it as a subordinate clause modifying the previous non-defining relative clause, but I now think that the word 'save' is a preposition rather than a conjuntion, therefore starting the clause with a prepositional phrase.
What do other people make of this?
Many thanks in advance for any advice.