58 MISS C. A. EAISIN OlST CERTAIN [Feb. IQOI, 



(2) Probable Contact- Alterations.^ 



It would be interesting if we could trace exactly the distribution 

 of the more and the less modified types among the altered rocks of . 

 the Ardennes; but quarries are far apart, and the country between 

 is generally a rolling plateau, sometimes thickly wooded. The 

 modifications, however, are found chiefly over the ' zone of Paliseul,' 

 as Dumont termed the country extending from west of the Mouse 

 to east of Bastogne.^ In this tract the chief alterations occur 

 around certain centres (as, for example, Libramont, Bastogne) over 

 areas of a few miles. Thus the distribution of these rocks is such 

 as might be expected, if an important igneous mass occurred below 

 the surface. 



Further, we can trace progressive alteration in specimens col- 

 lected in certain districts, and sometimes even in those from a single 

 quarry. For instance, some rocks with faint spots (as from near 

 St. Pierre) show under the microscope a matted, minutely crys- 

 talline mass of quartz or secondary felspar and white mica with 

 some biotite. In a more gritty band (from near Mont St. Etienne 

 west of Bastogne) the quartz and felspar-grains are coarser, sur- 

 rounded by minute greeaish films with some graphite, and the 

 flakes of biotite are larger and clustered. Other types have more 

 numerous or more scattered films ; or more abundant biotite, in nests 

 and groups. In others, streaks of biotite and associated minerals 

 (some sphene, iron-oxide, etc.) give the grit a banded appearance. 

 In all these, the micromineralogical change is not greater than is 

 often found in pressure-modified rocks ; but the microliths are not 

 connected with pressure-planes, and the development of biotite 

 (especially in clusters) is suggestive of contact-alteration. It has 

 the indented or interrupted form of, and a general similarity in colour 

 and appearance to, the mica in undoubted contact-rocks.^ 



The change in the rocks described above is slight and chiefly 

 traceable under the microscope, but previous observers have obtained 

 stronger evidence. Thus Yon Lasaulx found a granite in the Hohe 

 Yenn beneath upheaved Cambrian strata.^ Although the granite 

 is not exposed in other localities, yet at many places veins occur of 

 quartz, sometimes with felspar and mica, such as would often be 

 connected with a granite-mass. Further, M. Dupont obtained from 

 near Libramont crystals which he identified as doubtless chiasto- 

 lite, and this would be considered, as he points out, certain evidence 



^ Excluding the garnetiferous and hornblendic rocks. 



^ The ottrelite-rocks of Salm, and the granite of Lamersdorf in the Hohe 

 Yenn, are from other and distant localities. 



^ Such as those from Grlendalough (Ireland), slices from which were kindly- 

 lent me by Prof. Bonney ; or those from Andlau in the Yosges. 



^ The railway-cutting where this was exposed is grassed over, and I could 

 not examine the adjoining schists described by Prof. Gosselet. Close by, near 

 the bend of the road, is quarried a pale brownish muddy rock containing grains 

 of quartz, possibly fragmental, but the mass is quite decomposed. 



