56 MISS C. A. EAISIN ON CEKTAIF [Feb. ipOlJ 



investigation) I visited the district in 1898, and collected many- 

 specimens. After careful stndy of these, I made a second expedi- 

 tion in the summer of 1899, and gained observations over a wider 

 area of country. 



In the course of this investigation I have received much valuable 

 help from Prof. Bonney, who has examined all the specimens, 

 and has made many important suggestions. I have had also the 

 advantage of the loan from his collection of many slides of rocks, 

 from this and other localities, for comparison. 



The rocks of the district are classed as Cambrian and Devonian. 

 The former, according to the map, occupies four patches : — two 

 larger, Stavelot and the Mouse ; two smaller, Serpent and Rocroy. 

 The Cambrian rocks are generally blackish, and include shaly 

 micaceous grits, shales, slates, and strong compact rocks, often 

 quartz-veined, which in the hand-specimen resemble quartzites, but 

 under the microscope have the appearance of a quartz-grit. The 

 different species sometimes graduate one into the other, as along 

 the road at Trois Fonts, where they alternate repeatedly, while they 

 form a much-folded series by the railway- station below. Under 

 the microscope, we see scattered mica, partly or wholly secondary, 

 within the gritty groundmass, and often squeezed dark carbonaceous 

 streaks. 



The Lower Devonian slates and grits extend over wide areas of 

 the country, exhibiting at places alterations which I shall describe 

 later. Lithologically they bear much resemblance to the Cambrian 

 with similar micromineralogical change, a resemblance which is 

 found also in Britauny in the ' gres feldspathiques," the ' gres 

 armoricains,' and in the later (Devonian) rocks. 



II. MiNEKAL MODIFICATIOIS^S. 



(1) Results of Pressure. 



The district of the Ardennes is familiar to all geologists as an 

 example, both in Cambrian and Devonian, of the results of 

 pressure, which has caused a very general slaty cleavage, while 

 foldings and overthrust-faults have been described and figured from 

 many places. 



A much-crushed quartz-felspar rock occurs at several localities, 

 as at Lamersdorf, south of the village, where it is described by 

 A. von Lasaulx.^ In these quarries the base of the series consists of 

 a dusty pale-banded argiJlite followed by a layer, containing large 

 fragments of quartz, which is almost certainly a grit, although the 

 quartz has some resemblance to a broken-up vein. Bosses of the 

 quartz-felspar rock succeed, and it is found in a road-cutting on the 

 hillside below. All these rocks exhibit an imperfect cleavage dipping 

 roughly south-eastward. On examination with the microscope, we 

 see in all of them fragments of quartz dispersed in a groundmass 



1 Verhandl. Naturhist. Ver. Preuss. Eheinl. vol. xH (1884) pp. 445-48. 



