Correspondence. 229 



be in those conclusions, that they were not warranted by the evidence 

 which he has laM before the geological public, who will, I think, 

 agree with me in returning the Scotch verdict of '' not proven," and 

 advise him to try again if he wishes to convince the geological 

 world of their correctness.^ 



To avoid extending these remarks to too great a length, I will, in 

 conclusion, only refer to one more point in Mr. James Geikie's 

 reply. 



That gentleman differs from me as to the meaning of the term 

 "greywacke " in petrology,^ and in page 178 informs the readers of 

 your Magazine that " the greywackes familiar to Scottish geologists 

 do not • consist essentially of seventy-five per cent of quartz,' nor 

 have they any definite composition whatever. The term ' greywacke,' 

 as used by Scottish geologists, is applied exclusively to the hardened 

 felspathic, and sometimes argillaceous sandstones of the Silurian 

 regions, in which, although quartz is frequently present, it is by no 

 means a necessarily preponderating ingredient." 



Always regarding science and its nomenclature as cosmopolitan, 

 I am of opinion that such style of argument should be protested 

 against; as no doubt Mr. James Geikie would do, if informed that 

 he must be quite wrong, because " Manx " ^ geologists entertained a 

 totally different opinion of the rock species " greywacke." 



* Since my former communication, the arrival of Dr. Sterry Hunt in this country 

 has procured me the pleasure of his personal acquaintance. The opportunity thus 

 afforded xis, of comparing notes on chemical geology, showed how many similar con- 

 clusions we had respectively come to, from the study of widely different parts of the 

 globe, and assured us that any difference in opinion could not arise as to the agencies 

 employed in Nature's operations, although we might be somewhat at variance as to the 

 precise extent to which each agent had been engaged. 



* A study of the rock in the field in localities specially characteristic, combined with 

 an examination of the descriptions given by the numerous Avriters on the subject, has 

 resulted in my defining this rock species as follows : — 



Greywacke. — A sedimentary rock usually of a greyish colour (whence its name) 

 found extensively developed in the earlier geological formations ; but not specially 

 characteristic of anyone of same. Petrologically, "greywacke " is an impure sandstone, 

 more or less argillaceous, formed from the debris of previously existing rocks, re- 

 arranged by aqueous action, and subsequently, more or less eonsolidated. Usually 

 compact, it may vary in texture from fine-grained to coarsely conglomeritic ; the 

 stratification of the beds is frequently indistinct, unless viewed upon the large scale ; 

 when coarse it may contain fragments of fossils, and of other rocks, as clay-slate, 

 mica schist, granite, porphyry, limestone, etc. Mineralogically, it is essentially 

 quartz, with more or less clay, and frequently contains grains or scales of mica, 

 chlorite, talc, lithomorge felspar, calcite, iron pyrites, etc. Chemically, it is composed 

 of some seventy to eighty-five per cent, silica, along with alumina and a little oxide 

 of iron, with but traces of the alkali and alkaline earths. Chemical and micro- 

 scopical examination show comparatively little combined silica, the major part being 

 in the free state as quartz. 



This definition I maintain is in accordance with the views of all the writers on the 

 subject whom I have consulted, and in corroboration thereof, I would cite the follow- 

 ing references: Bischoff, iii. p. 132; Blum, p. 284; Brongniart, pp. 123, 126; 

 Coquand, p. 238; Cotta, p. 301; D'Halloy, p. 15 ; Erdman, p. 173; Grimm, p. 215; 

 Jameson, p. 226; Kjerulf, p. 72; liConhardt, p. 171; Mayer, iii. p. 1; Maccul- 

 loch, p. 358; Page, p. 309 ; Pinkerton, i. p. 291 ; Phillips, p. 654 ; Roth, p. 69; 

 Senft, p. 332 ; Zirkel, ii. p. 594. 



3 And our little Island is probably one of the best localities in Europe for the study 

 of this rock. 



