Ch. VIII.] cheonology of eocks. 89 



the gravel. In one case an intermediate shelf appears for a short distance 

 (three quarters of a mile) on the face of the mountain called Tombhran, 

 hetween the two upper shelves, and is seen nowhere else. It occurs where 

 there was the longest space of open water, and where, perhaps, the waves 

 acquired a greater than ordinary power in heaping up detritus. 



Next as to the precise horizontally of level maintained by the parallel 

 roads of Lochaber over an area many leagues in length and breadth, this 

 is a difficulty common in some degree to all the rival hypotheses, whether 

 of lakes or glaciers, or of the simple upheaval of the land above the sea. 

 For we cannot suppose the roads to be more ancient than the glacial 

 period, or the era of the boulder formation of Scotland, of which I shall 

 speak in the eleventh and twelfth chapters. Strata of that era of marine 

 origin containing northern shells of existing species have been found at 

 various heights in Scotland, some on the east and others on the west 

 coast, from 20 to 400 feet high ; and in one region in Lanarkshire not 

 less than 524 feet above high-water mark. It seems, therefore, in the 

 highest degree improbable that Glen Roy should have escaped entirely 

 the upward movement experienced in so many surrounding regions, — a 

 movement implied by the position of these marine deposits, in which the 

 shells are almost all of known recent species. But if the motion has 

 really extended to Glen Roy and the contiguous glens, it must have up- 

 lifted them bodily, without in the slightest degree affecting their horizon- 

 tality ; and this being admitted, the principal objection to the theory of 

 marine beaches, founded on the uniformity of upheaval, is removed, or is 

 at least common to every theory hitherto proposed. 



To assume that the ocean has gone down from the level of the upper- 

 most shelf, or 1250 feet, simultaneously all over the globe, while the land 

 remained unmoved, is a view which will find favor with very few geolo- 

 gists, for the reasons explained in the fifth chapter. 



The student will perceive, from the above sketch of the controversy re- 

 specting the formation of these curious shelves, that this problem, like many 

 others in geology, is as yet only solved in part ; and that a larger number 

 of facts must be collected and reasoned upon before the question can be 

 finally settled. 



CHAPTER VIII. 



CHRONOLOGICAL CLASSIFICATION OF ROCKS. 



Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically — 

 Lehman's division into primitive and secondary — "Werner's addition of a tran- 

 sition class — Neptunian theory — Hutton on igneous origin of granite — How the 

 name of primary "was still retained -for granite — The term " transition," why 

 faulty — The adherence to the old chronological nomenclature retailed the 

 progress of geology — New hypothesis intended to reconcile the igneous origin 

 of granite to the notion of its high antiquity — Explanation of the chronological 

 nomenclature adopted in this work, so far as regards primary, secondary, and 

 tertiary periods. 



