PART I.—REVIEW OF THE DIFFERENT THEORIES REGARDING THE FORMATION OF THE HILL. That remarkable group of hills, crags, and valleys included in the Queen’s Park of Edinburgh, and often collectively termed Arthur’s Seat, has been a fruitful source of interest and discussion to geologists. Charles Maclaren, in his ‶Geology of Fife and the Lothians,″ has made the hill classic ground to the geologist by his clear, intelligent, and carefully detailed description of the interesting geological phenomena observable. Several eminent geologists have made Arthur’s Seat the subject of their study and remarks since the publication of the first edition of ‶The Geology of Fife and the Lothians″ in 1838; but amongst them a diversity of opinion is expressed in regard to the age of the different deposits of igneous rock of which the hills are made up. Professor Archibald Geikie adopted the view held by Maclaren, in the first edition of ‶The Geology of Fife and the Lothians,″ that the igneous rocks of Arthur’s Seat belonged to two widely separate geological epochs; whilst another eminent geologist, Professor Judd, maintains a view, apparently adopted by Maclaren in the second edition of his work, published in 1866, that the whole of the rocks of Arthur’s Seat belong to one geological period. Lastly, we have a paper by Professor Bonney, read before the Geologists’ Association last year, comparing both theories, and giving some of his own observations and opinions on the subject. As the opinion of such eminent geologists must be of interest to