Geological Society of London. 435 



the quartz that I have not yet detected any in the felspar or mica ; 

 they are certainly extremely rare in these minerals, if they occur at 

 all. This, if established, would indicate a difference in the condi- 

 tion under which the minerals were formed, a point which I believe 

 has not yet received attention. 



For an account of the curious spontaneous movements of the fluid 

 in some of these cavities, and for other interesting matter connected 

 with the subject, I must refer you to Mr. Sorby's paper already quoted. 



During the past summer and autumn (1867) I have collected speci- 

 mens of the igneous rocks of the Midland coalfields from the follow- 

 ing localities : — Kinlet and Shatterford, west of Kidderminster ; the 

 Glee Hills, Little Wenlock, near the Wrekin, in Shropshire ; Coal- 

 ville near Bardon Hill, in Leicestershire ; and Matlock, in Derbyshire. 



A microscopical examiaation of thin sections shows that all these 

 rocks belong to the same type ; they do not differ more from each 

 other than do different specimens of any one of them. The toadstone 

 of Derbyshire is merely an amygdaloidal variety. 



The rocks of the Warwickshire coal-field differ considerably from 

 the foregoing. They contain hornblende instead of augite, and are, 

 therefore, true greenstones or diorites. They may be readily examined 

 in the railway cutting near Nuneaton, and also a little to the west of 

 Atherstone. All the rocks just enumerated are clearly older than 

 the surrounding Permians, which are never penetrated by them. 



Having now made upwards of four hundred sections of rocks and 

 minerals, I am inclined to believe that the following results of 

 microscopical examination will stand the test of further study : — 



1. The mineral constituents of the melaphyres and other fine- 

 grained igneous rocks may be determined with certainty, a result 

 which has not been attained by any other method of examination. 



2. The mineral constituents of the true volcanic rocks, and those 

 of the old melaphyres, are generally the same. 



3. The old rocks have almost invariably undergone a considerable 

 amount of alteration, and this change alone constitutes the difference 

 now existing between them and the more recent volcanic basalts. 



The basaltic lavas of the Rhine and Central France are composed 

 of a triclinic felspar, augite, magnetite, olivine, and frequently 

 apatite; the same minerals as those constituting" the old rocks above 

 described. I have fine-grained specimens of the latter hardly dis- 

 tinguishable from recent basalts ; and a section of dolerite from the 

 Puy de Barnere, in Auvergne, does not differ in any important par- 

 ticular from coarse-grained specimens from Eowley. 



It would be easy to extend the parallelism to other classes of 

 rocks, but I will now only observe that we have here another proof 

 of the doctrine long taught by LyeU — the uniformity and continuity 

 of the laws of Nature. 



Geological Society of London. — Concluded from the August Num- 

 ber, p. 394. — June 22nd, 1870. — " On the Physics of Arctic Ice as ex- 

 planatory of the Glacial Eemains in Scotland." By Dr. Eobert Brown, 

 F.E.G.S., etc. Communicated by Professor Eamsay, F.E.S., F.G.S. 



