In a letter1 dated August 9th 1838 to his geologist friend Charles Lyell Charles “Gas” Darwin wrote (italics mine):
You will be amused at some of the ridiculo-sublime passages in the papers, and no doubt will feel acutely a sneer there is at yourself.I suppose Darwin meant "ridiculously sublime". He was referring to the papers a friend of his had published in the Entomological Transactions, which he was sending to Lyell. Either Darwin didn't write his friend's name or it was taken out by his son Francis who edited the letters.
Not surprisingly, the only place where Darwin's idiosyncratic phrase "ridiculo-sublime" appears on the Internet is in his letter.
Note added on 14 March 2009: I have found the complete text of the letter in Darwin Correspondence Project. Darwin was referring to Frederick William Hope (see footnote 11). It was probably Francis Darwin who had deleted his name from the version included in The life and letters of Charles Darwin.
1The letter is on p. 295 of volume 1 of The life and letters of Charles Darwin, including an autobiographical chapter edited by his son Francis Darwin (1887). Full text is here.