344 H. J. Loice — Rock Diferentiation. 



by the decay of solid basalt in sitxi, and that the so-called ' volcanic 

 bombs ' in reality afford the best evidence of this decay, is, after all, 

 but an echo from the thunders of the old Neptunist controversy. 

 Long before Messrs. Tate and Holden wrote, the Rev. Dr. AY. Richardson, 

 a determined opponent of the Vulcanists, stated, in a memoir sent 

 to the Royal Society of Edinburgh, that the red strata had "been 

 once pure basalt." This memoir does not seem to have been published, 

 but the argument is emphasised in a paper printed in 1803 ^ and 

 in several subsequent publications. The most important of these is 

 Richardson's " Letter on Zeolite and Ochre," - in which he rightly 

 denies that the ochres ai'e " liozzolana or tiifo,''^ and quotes St. Fond in 

 support of his opinion. On p. 14 he says, "we find this ochreous 

 substance in its natural situation in every intermediate stage between 

 sound blue basalt and ochre red as minium ; and we see that the 

 passage from one extreme to the other is by shades perfectly 

 insensible." He does not like to say whether the change into ochre, 

 the cause of which he does not specify, has now come to an end or 

 is still in progress. 



Rerger and Conybeare ^ describe the bole as occurring in beds, but 

 do not commit themselves as to its mode of origin. It is fairly clear 

 that Richardson's uncompromising opposition to Hutton's theory of 

 terrestrial changes, and his denial of the existence of ash-beds in 

 association with true basalts, led to the rejection of his views on the 

 relations of the red zone of Xorth-East Ireland. These views, however, 

 prove to be far better founded than those maintained a century later 

 by the successors of the Vulcanist school. 



u 



III. A EEMAEKABLE INSTANCE OF RoCK DrFFERENXIATION. 



By Haefokd J. Lowe, F.G.S. 



(PLATE XIII.) 



PON Sheet 339 (Devonshire) of the new Geological Survey maps, 

 one instance out of the very numerous outcrops of igneous rocks 

 thereon indicated proves to be of unusual interest by reason of its 

 peculiar constitutional modifications in different parts of the same 

 mass. The rock in question occurs about four and a quarter miles 15° 

 north of west from Newton Abbot, near to the hamlet of Bickington, 

 within the limits of a farm named Lurcombe. It is an intrusive 

 amidst the shales and grits of the Culm, occurring almost on the 

 junction-line between that series and the Devonian, whose massive 

 limestones and volcanics dominate it in elevation within a quarter of 

 a mile on the south-east. 



1 " Inquiry into the consistency of Dr. Hutton's theory, etc.," Trans. R. Irish 

 Acad., vol. ix (1803), p. 4.58. See also "On the alterations . . . in the Structure 

 of Rocks, on the surface of the basaltic Country in the counties of DeiTy and 

 Antrim," Phil. Trans. Roy. Soc. (1808), pp. 195 and 200. 



2 Appendix I, p. 11, in Dubourdieu, " Statistical Survey of the County of Antrim," 

 Dublin, 1812. 



* " On the Geological Features of the North-Eastern Counties of Ireland," Trans. 

 Geol. Soc. London, Vol. iii (1816), p. 186. 



