He started, towards the end of June, on his expedition to Glen Roy, of which he writes to Fox: "I have not been very well of late, which has suddenly determined me to leave London earlier than I had anticipated. I go by the steam-packet to Edinburgh,—take a solitary walk on Salisbury Craigs, and call up old thoughts of former times, then go on to Glasgow and the great valley of Inverness, near which I intend stopping a week to geologise the parallel roads of Glen Roy, thence to Shrewsbury, Maer for one day, and London for smoke, ill-health and hard work."
He spent "eight good days" over the Parallel Roads. His Essay on this subject was written out during the same summer, and published by the Royal Society. ('Phil. Trans.' 1839, pages 39-82.) He wrote in his Pocket Book: "September 6 [1838]. Finished the paper on 'Glen Roy,' one of the most difficult and instructive tasks I was ever engaged on." It will be remembered that in his 'Recollections' he speaks of this paper as a failure, of which he was ashamed.
At the time at which he wrote, the latest theory of the formation of the Parallel Roads was that of Sir Lauder Dick and Dr. Macculloch, who believed that lakes had anciently existed in Glen Roy, caused by dams of rock or alluvium. In arguing against this theory he conceived that he had disproved the admissibility of any lake theory, but in this point he was mistaken. He wrote (Glen Roy paper, page 49) "the conclusion is inevitable, that no hypothesis founded on the supposed existence of a sheet of water confined by BARRIERS, that is a lake, can be admitted as solving the problematical origin of the parallel roads of Lochaber."
Mr. Archibald Geikie has been so good as to allow me to quote a passage from a letter addressed to me (November 19, 1884) in compliance with my request for his opinion on the character of my father's Glen Roy work:—
"Mr. Darwin's 'Glen Roy' paper, I need not say, is marked by all his characteristic acuteness of observation and determination to consider all