70 MISS C. A. EAISIN ON CEETAIN [^^b. I9OI, 



of carbonaceous material. Yet the recrystallized groundmass often 

 resembles an early stage in contact-altered rocks. It frequently 

 contains some of the characteristic biotite, which sometimes may 

 be abundant even in a rock crowded with hornblende-tufts. 



Thus we have to account for small limited patches often compa- 

 ratively undisturbed, showing gradual alteration, with a marked 

 line of change at one part, yet without any exposure of an igneous 

 rock to cause the modification. I believe that the true solution will 

 be found in the suggestion, made by Prof. Bonney in 1890,^ that we 

 see here results due to hot springs. They would modify the 

 sedimentary strata, so that we should expect to trace a gradual 

 transition. At the same time, a line of division would be marked 

 around the central mass, often crossing the banding : just as a 

 deposit from ordinary infiltration may end almost abruptl}'. The 

 alteration would occur over limited areas,^ which might appear in 

 section as ' nodules,' or lenticular patches, or partial bands. 



Mineral differences are sometimes exhibited along zones in the 

 nodules, or along bands in the altered layers. This might be due to 

 the marginal effects of the heated water. Even in a lava-flow 

 minerals may be sublimed and deposited at one part of the layer. 

 Thus Scrope pointed out that 



' the specular iron, so frequently met with in lava-rocks, is e'ridently sublimerl 

 by [the intense heat of the lava] and .... is always found in the upper parts 

 of the bed or current ; while the lower parts of the rock .... have obviously 

 lost all or the greater part of their iron by sublimation .... Or, as in many 

 of the currents of Etna, the upper parts .... contain much specular iron, ..... 

 the lower and compacter division abounds in magnetic iron, in grains or 

 octahedral crystals.' (' Volcanos ' 2nd ed. 1862, p. 144.) 



In the Ardennes ' nodule ' the central and main part contains 

 generally garnets, and often ottrelite, and is crowded with graphite. 

 The last named substance (abundant only here) might have been 

 deposited from the decomposition of a hydrocarbon contained in the 

 waters of a hot spring.^ 



The peculiar character of the garnets (so different from those 

 ordinarily found in schists or even in igneous rocks), their very 



^ ' On the Crystalline Schists & their Relation to the Mesozoic Rocks in the 

 Lepontine Alps ' Quart. Journ. Geol. Soc. vol. xlvi (1890) p. 214 note. 



^ Compare the cherty nodules of Stotfield which Prof. Judd attributes tc 

 ' purely chemical agencies,' Quart. Journ. GeoL Soc. vol. xxix (1873) p. 136. 

 In Cornwall patches of altered rock occur, which also may be compared. Here 

 the tourmaline, doubtless a result of infiltrating waters, ha& the same inter- 

 rupted granular appearance as the biotite or hornblende; like these, it is 

 sometimes developed along lines, and commonly forms tufts. I am indebted 

 to the authorities of the Natural History Museum for facilities of studying 

 slides from the Mousehole, Penzance. 



^ Just as the hot springs of California have deposited silica and sulphides of 

 metals in accretions or disseminated through the rock, as described by J. A. 

 Phillips, Quart. Journ. Geol. Soc. vol. xxxv (1879) p. 390. See also Geol. Surv. 

 California, vol. i (1865) pp. 92, 94. In slides at the Natural History Museum, 

 belonging to the collection of that author, from a rock of Steamboat Springs 

 (Nevada), an opacite-dust is scattered, reminding me of the distribution of 

 carbon in the Ardennes rocks, although in the Nevada slide the opacite is 

 probably a metallic ore. 



