ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXV 



The microscopic researches of Mr. Sorby on the structure of fresh- 

 water marls and Umestones open out a new field for inquiry as yet 

 little more than indicated. The idea of ascertaining the origin of 

 the structure through a determination of the forms of the minute 

 particles into which shells resolve themselves by decay, and of esti- 

 mating the relative proportions of the microscopic ingredients of a 

 rock by delineating on paper the outlines of the particles present in 

 a thin section of the stone with the aid of the camera lucida, then 

 cutting them out and weighing the figures of each kind separately, is 

 a process I believe wholly new in geological research and due to our 

 ingenious associate. The value of the proceeding may be tested by 

 the results, which, so far as they are published, are excellent. So 

 long as the microscope thus employed is guided by a practical 

 geologist, our science will be a. gainer by this kind of investiga- 

 tion. 



The distinction of all granites into two species or varieties, each 

 characterized by mineralogical and geological peculiarities, has been 

 forcibly insisted on by M. Delesse, and illustrated from his researches 

 among the rocks of the Vosges mountains. He distinguishes, — 1st, 

 the ' granites des Ballons,' containing little quartz, orthose in large 

 crystals, felspar (of the 6th sj^stera), dark mica affected by acids, and 

 frequently hornblende, ordinarily accompanied by sphene ; and, 2nd, 

 the ' granite des Vosges,' mainly made up of quartz and orthose, with 

 the addition of a little felspar (of the 5th system), dark mica affected 

 by acids, and transparent mica in smaller quantities not affected by 

 acids. This granite often takes a gneissoid structure. The former 

 kind is eruptive, and constitutes the more elevated portions of the 

 granitic chain ; the latter has rather the characters of a metamorphic 

 rock. The distinction between the two sorts is not merely local, and 

 has been observed by M. Delesse in not a few granitic localities ; 

 among other regions, in Ireland. 



In a memoir on the mineralogical and chemical constitution of the 

 rocks of the Vosges, M. Delesse discusses those phsenomena of me- 

 tamorphism characterized by felspathization, that is, by the develop- 

 ment of crystals of felspar (of the sixth system) in ancient stratified 

 rocks. To these felspathised rocks of the Vosges he applied the 

 name Grauwacke, a term by which he proposes to designate every 

 sedimentary rock, whatever be its age or structure, in which crystals 

 of felspar of the sixth system have become developed. I am in- 

 clined to object to the revival of the name Grauwacke in the present 

 stage of geological research ; it has been used so variously, loosely, 

 and indefinitely that it had better be wholly dropped from our no- 

 menclature. The sense in which it is used by M. Delesse is not that 

 in which the majority of geological writers have employed it, and 

 since the class of rocks to which he would restrict the name are 

 highly important and well deserving of specific distinction, the in- 

 vention of a new term would not only have been excusable, but also 

 of good service. The question may arise whether the apparent fel- 

 spathization, in the sense in which this word is used by M. Delesse, 

 may not in some instances rather depend upon the original diffusion 



