^{page 50 note 1} British Association Reports, 1865. Transactions of Sections, p. 53.Google Scholar

^{page 50 note 2} Geologists have not as yet learned to appreciate the true value of microscopic investigation, In very many cases the simple examination of a rock section can at once determine whether a rock is a true igneous rock, or the product of a secondary metamorphism. The admirable researches of Mr. Sorby show how much may be effected in geological research by the use of the microscope. The writer, fully appreciating this, has devoted much time and labor since 1851 to this branch of petrology, and has now above 900 sections of crystalline and metamorphic rocks from about 480 localities, in different parts of the world.Google Scholar

^{page 50 note 3} Mohr in his Geschichte der Erde, 1866, p. 516, gives as No. 92 of his Theses, “Geologus non aestimatur ex calceis in peregrinationibus detritis;” it is admitted that some geologists of repute are neither Palæontologists nor Petrologists, nor much at home in either mineralogy, chemistry or mathematics: it becomes, therefore, somewhat perplexing to define what constitutes a geologist.Google Scholar

^{page 53 note 1} When it is remembered that a single accurate ehemioal analysis of such a rock, which shows hut a few lines in print, represents a value of fcom five to ten guineas in actual labour and expense to the chemist, it is easy to understand how, in general, so much appears in print about the ideal composition of rocks, and so little verification of the same by actual chemical analysis.

^{page 53 note 2} The italics are sic in memoir.

^{page 54 note 1} A hydrate of magnesia is also known as brucite.

^{page 55 note 1} A rock which, it must be remembered, consists essentially of seventy-five per cent of quartz.

^{page 56 note 1} Classification des Roches, p.63.Google Scholar

^{page 58 note 1} The writer takes the opportunity of here stating, that the results of a protracted study of this subject (several of the details of which will be found in a series of communications, which hare appeared at intervals, since 18S3, in the Magazinfdr Naturvidenskab, Skandinaviske ftaturforskeres Forhandlinger, Philosophical Magazine, Quarterly Journal of the Geological Society, Edinburgh Philosophical Journal, and other periodicals), have induced him to conclude:—

(1.) That when the geological epoch of the appearance of an eruptive or intrusive rock is known, such rock will be found to differ essentially, in mineral constitution, from similar rocks injected at a different geological period.

(2.) That eruptive or intrusive rocks of identical mineral constitution, have made their appearance or intrusion into the earth's crust at the same geological epochs.

(3.) That the minerals, or classes of minerals, accompanying or associated with such intrusive or eruptive rocks, may serve as a means of distinguishing the several eruptions in geological chronology, in a manner analogous to the determination of sedimentary strata, by the fossils, or classes of fossils, which they may be found to contain.

^{page 58 note 2} The views in question are far from being new, for under various dresses they appeared, were discarded, and re-appeared from time to time, even from the infancy of geology; some of their advocates having even gone so far as to suppose that the state of granite was one which all rocks would ultimately arrive at, after undergoing a process of slow but constant internal alteration or fermentation.