PREFACE.
It is now four years since the notes were written on which this little book is founded. I read an abstract of it at the meeting of the British Association, at Southampton, in 1846; another abstract was read at the Geological Society, in November, 1847. In consequence, however, of its length, and of its being not entirely composed of original observations, it could not be published in extenso in the proceeding's or transactions of either of those bodies. I have since frequently intended to publish it myself, in order that the labour I expended on its compilation might not be entirely thrown away; but have been too much occupied with the duties of the Geological Survey of Great Britain, to which I am now attached, to spare the necessary time for its completion. Buried for the most part in the heart of the Welsh mountains, I have been unable to do more now than take advantage of some rainy days this spring for revising my original rough notes. I mention these facts in some measure to excuse the incompleteness of many parts, and the hastily-written style of all, and trust the reader will pardon me accordingly should his eye or his ear be offended.
