Vol. 50.] CRYSTALLINE SCHISTS IN THE LEPONTLNE ALPS. 287 



II. The Altkirche Marble. 



The belt of sedimentary rocks in which this marble occurs extends 

 from a point on the western slopes of the Oberalp Pass along the 

 valley of the Upper Peuss and across the Purka Pass into the valley 

 of the Upper Rhone. On the latter pass and to the east it is 

 bounded, on the northern side, by a more or less micaceous gneiss, 

 on the southern usually by a group of greenish schists, which are 

 followed by the micaceous schists or gneiss of the northern slopes 

 of the St. Gothard Pass. These greenish schists will be referred to 

 (for brevity) as the ' Hospenthal schists.' In this communication I 

 shall abstain from discussing the question of their origin, merely 

 remarking that as a rule they have been greatly affected by pressure 

 and that I rank them among the crystalline rocks of the Alps. On 

 the slopes, however, of the Oberalp Pass, as will be indicated below, 

 they do not immediately succeed the above-named belt. Its rocks 

 (with the exception of the marble) are generally dark in colour 

 (from a dull lead- blue to almost black) ; the marble varies from 

 white to light grey. The belt of sedimentary rocks crops out 

 usually on the northern slopes of the valley : good sections are not 

 common, so much being concealed by debris and turf. 



I purpose to deal with the subject by describing a series of sections 

 (the best which I could find) from east to west, over a distance 

 of about 11 miles in a straight line. In order to make some 

 approach to brevity I shall suppress all minor details of structure 

 and mineral composition, and content myself with saying that every 

 important point in regard to similarity or dissimilarity has been 

 carefully tested by microscopic examination of specimens collected 

 for that purpose. As there can be no doubt that the rocks of this 

 belt, in which the marble occurs, whatever be their geological age, 

 are sedimentary in origin, I will refer to it, for brevity, as the 

 ' sedimentary belt.' 



(a) Section on the higher eastern slopes of the Oberalp. — This 

 section has been recently exposed, in the construction of a military 

 road leading up the mountain on the northern side of the Oberalp 

 Pass. That road cuts obliquely across the sedimentary belt at angles 

 of 20° or 30° with the general strike of its rocks, and enters on the 

 northern mass of micaceous gneiss some 1600 feet above Andermatt. 1 

 The belt consists, on the northern side, of a rather dark phyllite, 2 

 interstratified with hard sandy bands, which gradually becomes 

 more calcareous as we proceed southwards. In this part paler and 

 more crystalline-looking layers (say from an inch downwards), which 



1 The measurements in this paper were taken with a pocket-aneroid, and 

 therefore they are only approximate. I think the error in reading would be less 

 than 10 feet, but the instrumental error is uncertain, as it varies from day to 

 day. 



2 I employ the term ' phyllite ' for a slaty rock in which an unusually large 

 amount of a minute secondary mica has been developed, which gives a peculiar 

 ' sheen' to the cleavage-surfaces, i. e. one step nearer a schist than a slate, but 

 still a long way from the former. 



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