316 MR. J. PARKINSON ON THE ROCKS OF [May 1900, 



felspar, forming a kind of network with the quartz, is principally 

 orthoclase with some microperthite. A zone of brownish unstriped 

 felspar usually surrounds the larger plagioclases. The section 

 contains a few flakes of greenish mica in various stages of decom- 

 position, and a rare zircon-crystal. This rock has forced its way 

 into a fine-grained, greyish or greenish, non-porphyritic diabase, 

 often shattering it to a wonderful extent. A thin section cut from 

 a specimen of the diabase, purposely chosen from a point where 

 it might be supposed to have escaped any alteration by the aplite, 

 affords an excellent example of the extent to which permeation 

 by the acid magma has taken place. The older rock in the hand- 

 specimen shows no sign of quartz ; it is compact, with the charac- 

 teristic weathering and fracture of a diabase. The microscope, 

 however, reveals the presence of a considerable quantity of quartz, 

 in grains, often forming micropegmatite with an acid felspar. 

 Recognizable augite in some quantity is found in the rest of the 

 section. No clearly-defined line of demarcation exists between the 

 acid and basic parts. On the contrary, the acid minerals have 

 insinuated themselves between the constituents of the diabase, and, 

 although often conspicuous, are sometimes only distinguishable with 

 difficulty. The quartz-grains do not exceed *036 inch in length; 

 the average length is about *016 inch. 



The presence of this mineral in so homogeneous-looking a rock 

 is not easy to understand, and I am indebted to Prof. Bonney 

 for the suggestion that the siliceous and felspathic substances have 

 been conveyed into the diabase in the form of an aqueous solution 

 rather than by an injection, pure and simple, of the acid magma. 



The recent work of M. Lacroix has an important bearing on 

 this point. In his paper on ' Le Granite des Pyrenees et ses 

 Phenomunes de Contact' he enunciates the theory to which his 

 observations lend support in the following terms: — 'Lesphenomenes 

 de contact des roches eruptives sont le resultat de la transformation 

 d'une roche preexistante, apportant sa caracteristique personnelle, 

 sous rinfluence d'agents mineralisateurs, le plus generalement 

 accompagues d'elements volatils ou solubles qui, en se fixant sur 

 la roche modifiee, en transforment plus ou moins completement la 

 composition chimique.' 1 



The zone of altered rock in contact with the intrusive granite 

 furnishes an example of the action to which he refers (op. cit. p. 8). 

 This zone is characterized by the presence of granular felspar which 

 the older rock (schists, quartzites, etc.) may acquire by one of two 

 methods, or by both of these acting together : — (i) by simple 

 injection of the acid magma, or (ii) by ' imbibition/ when microsco- 

 pical examination is necessary to establish the phenomena. The 

 second method he considers to be carried out by means of hydro- 

 thermal agents, or, in general, by ' agents mineralisateurs.' 



1 Bull. Serv. Carte Geol. France, No. 64, vol. x (1898-99) p. 48. See also 

 Michel-Levy ' Sur la Classification des Magmas des Eoches eruptives,' Bull. Soc. 

 Geol. France, ser. 3, vol. xxv (1897) p. 370 ; & Lacroix, Bull. Serv. Carte Geol. 

 France, No. 42, vol. vi (1894-95). 



