In my observations on the Laurentian Rocks of Birtain which appeared in the last number of the Geological Magazine, there is one statement which calls for midification, and another which I revoke. The striking discordance of direction or strike between the true Laurentian rocks of the North-Western Highlands and Islands, and the superficial strata of Cambrian and Lower Silurian age as described by me, is undoubtedly correct; but in another paragraph it is inadvertently said that the Silurian rocks of Britain trend everywhere from NE. to SW. For ‘everywhere’ the word ‘usually’ should have been employed, as there are tracrs wherein these rocks unquestionably range from W. to E.