A few words of introduction are called for to explain the circumstances under which this essay was written and the reason why its publication has been so long delayed.

The liberal founder of the Sedgwick Prize left to the examiners considerable discretion in the choice of subjects, but it has been their general custom to propose some line of enquiry in which it might fairly be expected that the opportunities for original research and facilities for carrying it on were not out of reach of the student who was fresh from his University training and who was as yet unfettered by any obligations as to his line of study; so that the work proposed might contain in it the suggestion of a career as well as a test of power of original investigation.

The scheme has so far been eminently successful, and almost every winner of the Sedgwick Prize has become an authority upon the branch upon which he was invited to write.

Thus Mr Roberts, who, as Assistant to the Woodwardian Professor, was acquainted with the rich stores of material available for research in the Woodwardian Museum, had his attention more particularly turned to the correlation of the Jurassic Rocks of the Northern district of England with those of the South-West.

This enquiry was attended with considerable difficulty from the fact that throughout the greater part of the period the deposits were laid down under locally shifting geographical conditions, so that the district was from to time divided into different and changing hydrographical areas, the sediment varying as barriers disappeared or were introduced and the forms of life more or less readily yielding to the influence of external circumstances.