Vol. 50.] OP CAEEOCK FELL. 319 



3. Minoe Textt/eal and Mineealogical Vaeiations. 



The remarkable differences, apparent to the most casual observer, 

 among specimens collected from the Carrock Fell gabbro area are 

 due to the coexistence of different kinds of variation, which must be 

 considered separately. I shall distinguish : — ■ 



(i) Minor variations in texture, and sometimes in mineralogical 

 constitution, usually on a small scale, following certain 

 directions of banding, or without any evident arrangement ; 



(ii) Wide variations in chemical composition, and consequently 

 in mineralogical constitution, having a definite relation to 

 the form of the intrusive mass as a whole ; 



(iii) Strictly local modifications, forming part of a reciprocal 

 metamorphism between (a) gabbro and enclosed masses of 

 lava, or (6) gabbro and granophyre. 



These will not all be discussed at equal length. The first and 

 second are jointly answerable for the great dissimilarity between 

 specimens collected from different spots, and of these the second is 

 of greater interest. 



I have already alluded to the variability in texture of the gabbro. 

 As seen in the field, the change from coarse to fine grain is often 

 rather abrupt. Sometimes the two are associated in a quite irregular 

 manner, or patches of coarser rock occur embedded in finer. Such 

 contrasts are seen not only on a small scale, but also between 

 adjacent portions of gabbro, perhaps 100 yards across. In other 

 places rocks of different textures are associated in alternating 

 thin layers, simulating stratification, and slight differences in 

 durability produce a fluted aspect on a weathered face. "Ward 

 appealed to this peculiarity in support of his theory that the gabbro 

 represents metamorphosed volcanic rocks, but such an idea is, for 

 many reasons, quite untenable. The phenomenon, indeed, is a very 

 common one, and must be familiar to most geologists who have 

 studied gabbros or other basic rocks. 1 It presents a rather 

 perplexing problem, and suggests that some factor not yet fully 

 appreciated has had some determining influence on the crystalliza- 

 tion of such rocks. The banded structure of rhyolites, where 

 crystalline or spherulitic layers alternate with glassy, has been 

 explained by Iddings 2 as depending upon the different proportions 

 of water contained in different parts of the magma, which were 

 drawn out in the direction of flow, but such an explanation could 

 have no application in the present case. I shall give evidence to 

 prove that the gabbro-magma had very little viscosity when it was 

 intruded, and that diffusion was able to operate through it after the 

 intrusion, so that the banded structure can scarcely be taken to 



1 For remarks on this point, see Gr. H. Williams on the gabbros of Mary- 

 land, Bull. 28 U.S. Geol. Surv. vol. iv. (1886) pp. 25, 26, and A. C. Lawson on 

 the anorthosites and gabbros of Canada, Neues Jahrb. Beil. Bd. viii. (1893) 

 pp. 448 et seqq. [See also the paper by Sir A. Geikie and Mr. Teall, which 

 is to be published in the present volume, where additional references are given.] 



2 Amer. Journ. Sci. ser. 3, vol. xxxiii. (1887) pp. 43-45. 



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