of the Caithness ichthyolites were blended, it was stated, with the fragments of a "supposed tortoise nearly allied to trionyx;" one of the ichthyolites, a _Dipterus_, was characterized by large scales, a double dorsal, and a one-sided tail; the entire lack of shells and zoöphytes was remarked, and the abundance of obscure vegetable impressions. In short, had the accomplished writer of the appendix been briefly describing the beds at Cromarty, instead of those of Caithness and Gamrie, he might have employed the same terms, and remarked the same circumstances--the striated nodules, the mineral tar, the vegetable impressions, the absence of shells and zoöphytes, the large-scaled, and double-finned ichthyolites--the peculiarities of which applied equally to the _Dipterus_ and _Diplopterus_--and the supposed tortoise, in which I once recognized the _Coccosteus_. It was much to know, that this doubtful formation--for as doubtful I still regarded it--was of such considerable extent, and occurred in localities so widely separated. I corresponded with the courteous author of the appendix, at that time General Secretary to the Northern Institution for the Promotion of Science and Literature, and Conservator of its Museum; and, forwarding to him duplicates of some of my better specimens, had, as I had anticipated, the generic identity of the Cromarty ichthyolites with those of Caithness and Gamrie fully confirmed. My narrative is, I am afraid, becoming tedious; but it embodies somewhat more than the mere history of a sort of Robinson Crusoe in Geology, cut off for years from all intercourse with his kind. It contains, also, the history of a formation in its connection with science; and the reader will, I trust, bear with me for a few pages more. Seasons passed; and I received new light from the researches of Agassiz, which, if it did not show me my way more clearly, rendered it at least more interesting, by associating with it one of those wonderful truths, stranger that fictions, which rise ever and anon from the profounder depths of science, and whose use, in their connection with the human intellect, seems to be to stimulate the faculties. I have often had occasion to refer to the one-sided condition of tail characteristic of the ichthyolites of the Old Red Sandstone. It characterizes, says Agassiz, the fish of all the more ancient formations. At one certain point in the descending scale Nature entirely alters her plan in the formation of the tail. All the ichthyolites above are fashioned after one particular type--all below after another and different type. The bibliographer can tell at what periods in the history of letters one character ceased to be employed and another came into use. Black letter, for instance, in our own country, was scarce ever resorted to for purposes of general literature after the reign of James VI.; and in manuscript writing the Italian hand superseded the Saxon about the close of the seventeenth century.