354 George P. Merrill — On Rock-iceathering . 



EXPLANATION OF PLATE XVI. 



Fig. 1. — Lindstromaster antiqua (Hisinger), 1837. 

 ,, la. — Outline of actinal surface of type -specimen, nat. size. 

 „ \b. — The same magnified 4 diameters. 



Bed C, corresponding to the Wenlock Shale, Gotland. 



,, 2. — PalcBasterina Bonneyi, sp. nov. 

 ,, 2a. — Abactinal view of specimen, magnified 2 diameters. 

 ,, 2b. — Actinal ,, ,, ,, 3 diameters. 



Ludlow Shales : Leintwardine, Shropshire. 



III. — A Discussion on the Use of the Terms Eook- weathering, 

 Sekpkntinization, and Hydrometamokphism. 



By George P. Merrill, 

 Head Curator, Dept. of Geology, U.S. National Museum, Washington, D.C., U.S.A. 



IN the abstract of a paper by Mr. Thomas H. Holland, read before 

 the British Association, Section C (Geology), Bristol, 189S, 

 printed in the January number of this Magazine,' on "The Com- 

 parative Actions of Subaerial and Submarine Agents in Eock 

 Decomposition," the author brings up again a question which the 

 present writer has often had occasion to face, and has vainly tried 

 to solve in a manner entirely satisfactory even to himself. 



The question relates primarily to the limitation of the term 

 weathering as applied to rocks, and indirectly to the general subject 

 of hydrometamorphism and metasomatosis as manifested in the 

 production of serpentine. 



In my work on ''Eocks, Eock-weathering, and Soils" (p. 174), is 

 made the following statement : — 



" The term weathering, as here used, is applied only to those 

 superficial changes in a rock-mass brought about through atmospheric 

 agencies, and resulting in a more or less complete destruction of the 

 rock as a geological body, as where granitic rocks are resolved into 

 sand and kaolinic material, with liberation of carbonates of the 

 alkalies and of lime, and oxides of iron. It does not include 

 those deeper - seated changes taking place below the zone of 

 oxidation and which result mainly in hydration and the production, 

 it may be, of new mineral species, as chlorite, sericite, zeolites, etc., 

 but during which the rock-mass as a whole I'etains its individuality 

 and geological identity. The distinction is not one that has been 

 sharply insisted upon, and, indeed, geologists and petrologists as 

 a rule have been extremely careless in their use of such terms as 

 alteration, decomposition, and weathering. The distinction drawn 

 here is essentially that made by Eotli (Allgemeine u. Chemische 

 Geologic) between Verwitterung and Complicirte Verioitterung. For 

 reasons above stated and others given on p. 161, it seems best to 

 limit the terms weathering and decomposition to processes involving 

 the destruction of the rock -mass as a geological body, and to 

 designate the purely mineralogical, deeper - seated changes, as 

 alteration, which may or may not be due wholly to hydrometa- 

 morphism." 



* Geol. Mag., January, 1899, pp. 30-1. 



