of the Lepontine Alps. 13 



constitution of the different beds was originally dissimilar, so that 

 the course and the results of any metamorphic action would neces- 

 sarily have varied in different parts of the whole complex. Further, 

 it is not necessary to assume that any chemical metamorphosis must 

 have embraced an entire complex of calcareous or dolomitic strata; 

 it might equally as well have been restricted to certain regions, 

 (marked off by stratification, fissure, or other lines) thus producing 

 heterogeneous mass-shaped intercalated deposits, such as for instance 

 gypsum and anhydrite imbedded in rauchwacke. As a matter of 

 course, these irregular intercalations are not then of marked value as 

 geological horizons. At the Airolo mouth of the tunnel, gypsum 

 or anhydrite only appeared as accessories in the quartzite beds. 

 No. 20 and 22, belonging to the dolomitic series, and in fissures in 

 the adjoining rocks. In the calcareous rocks of the Ursern Valley, 

 the occurrence of gypsum was restricted to lumps of alabaster im- 

 bedded in the clayey southern wall-rock. 



Having regard to the petrographic variety in rocks belonging to 

 one and the same calcareous series, it seems hazardous to ascribe, 

 a priori, a definite geological age to such rocks when they are met 

 with isolated. For this reason I have indicated on the geological map 

 of the railway line, by a single colour, all limestones ranging between 

 the Jurassic and Archgean, noting by index letters their special petro- 

 graphic characters (dolomite=D. ; rauchwacke:=R. ; marble=:M. ; 

 cipoline=C. ; calc-schist=Cas.), and leaving the question of their 

 exact geological range to future exploration. I have paralleled the 

 limestone series of the Ursern valley, which is considered to be 

 Jurassic, with the dolomitic series of the south side, in spite of 

 considerable petrographic differences — rauchwacke and saccharoidal 

 dolomites are, for instance, wanting in the tunnel below the Ursern 

 valley, whilst cipoline predominates there, though absent on the south 

 side, etc. The appearance of quartzitic beds in the foot-wall of the 

 Jurassic (Liassic) rocks affords a means of identifying them on both 

 sides of the St. Gothard. It has ah-eady been remarked in " Verhandl. 

 der Schweizer Naturfor. Gesellsch." 1874:-75, p. 139, and in the text 

 to the " Geol. Profile " (French, p. 47; German, p. 43) that beds of 

 quartzite (resp. sandstone) are regularly associated with the lime- 

 stone series north and south ; they occur, for instance, at the 

 Nufenen pass (a la Cruina), in the tunnel, near Lago Ritom, near 

 Prato, and, on the other side, on the Langisgrat, Furka, near Eealp, 

 and Altkirche. 



The description given by Prof. Bonney on p. 210 of his paper of 

 a section along a ravine in Val Canaria agrees in its principal features 

 with my own surveys in the same ravine and the adjacent areas, 

 which have been used for the construction of plate v. of the geological 

 map of the railway ; and on principle I cannot object to this author's 

 explanation of repeated identical beds (rauchwacke, in this case) by 

 faults instead of by folding, though I have based the construction 

 of this part of the map on the supposition of folds. The construction 

 of ideal folds on ideal sections means for me nothing but a way of 

 indicating the supposed identity of certain beds, of which only the 



