ON THE VARIOLITIC EOCKS OF MOXT GENEVRE. 301 



If we follow the stream of the Chenaillet Valley, we find it 

 turning- east under a clitf-set mountain-side to Clavicres ; and here 

 the third valley opens southward. This we have styled, for uni- 

 formity, the Gimont Valley, since Mt. Gimont forms its south-west 

 angle and the Col de Gimont (2428 metres) is at its head. Here 

 the dominant mass is Mt. La Plane, the cliffs and great taluses of 

 which form the western wall. On the east the slopes are far 

 smoother, with rounded gabbro-masses and blue serpentine showing 

 unmistakably at a distance of some miles. The Col de Gimont is 

 formed by an interesting ridge, scarped to the north, dipping 

 smoothly and steeply to the south, the boundary between Prance 

 and Italy running exactly along its crest. The rocky prominences 

 of Cima Saurel (2453 metres) occupy the south-east angle. 



Erom what follows it will be seen that the series of variolitic 

 rocks lies mainly between the Durance and the branch of the 

 Piccola Dora in the Gimont Valley, and may have an important 

 extension outside our area towards Cervieres on the south. Pro- 

 bably one of the most easily attained localities in which variolite 

 may be studied in situ is the north face of Mt. La Plane, just within 

 the Italian frontier, and a kilometre south of the highroad. The 

 whole east front of the same mountain forms one of the most admi- 

 rable fields for the collector. 



IV. Rocks found in the Gondkan Valley. 



As it is from the bed of the Durance that the principal supply of 

 variolite has been derived in the past, it is natural to make the first 

 search for the rock in the upper reaches of this stream. Proceeding 

 south from the village of Mont Genevre among the boulders strewn 

 over the bed, one soon finds plenty of material. Following the 

 stream up the valley, one notices, scattered over the limestone floor, 

 great blocks of coarse gabbro (euphotide) and variolite, some of the 

 latter reaching to 5 feet in diameter and being spherulitic through- 

 out. Amongst these, but numerically less important, arc blocks of 

 dense fine-grained diabase ; porphyry, with green saussuritic felspars ; 

 serpentine with or without diallage-crystals and cleavage-flakes ; 

 great masses of agglomerate containing variolite and diabase ; and 

 finally, dolomite, " cargneules," limestone, and limestone-breccia. A 

 closer scrutiny of the gabbro-blocks shows that some of this rock is 

 fine-grained, and some schistified with largo diallage- and felspar- 

 eyes : it shows no signs of the asserted passage into variolite, but is 

 traversed by many dykes of diabase as well as by felspathic segre- 

 gation-veins. 



Here in the stream we have specimens of all the rocks of the 

 district spread out before us : the dykes and agglomerates are sug- 

 gestive of volcanic origin, but of this there is no hint in the descri})- 

 tions of Prof. Lory, who records neither of these two rocks. This 

 author, moreover, regards the coarse gabbro, the fine-grained diabase, 

 and the porphyrite with green felspars, as only dift'erent degrees of 

 coarseness of the same rock, which he describes as euphotide, and of 

 which the serpentine and variolite are alike structural modifications. 



