Jura As I considered that the mineralogical history of Jura had already been amply detailed, I ascended its well-known mountains rather to gratify my love of the picturesque, than with the hope of acquiring any new ideas on the subject of its structure.

Yet as the predominant opinions seemed to have determined that the tract in question was composed of a granular quartz, and as it had been compared with the northern part of Scotland asserted to consist of the same rock, with the ridge of Schihallien, amply described as such by Professor Playfair, and with numerous other foreign rocks described by various authors under this designation, I had with others admitted the term, and the æra of formation which this term implied. My own researches having given me reason to differ in opinion from these authorities, I have thought it necessary to describe the appearances which I saw; the question of its rank in the great society of rocks can only be determined after all its circumstances have been fully investigated.

The whole island extending to a length of 30 miles, and varying in breadth from one mile to six, is fundamentally composed of this one rock, which is also to be found extending through Scarba on its northern, and through a great portion of Isla on its southern shore. The other rocks which occur, with the exception of an important series to be immediately described, either occupy small spaces apparently subordinate to the general mass, or consist of veins