396 



PKOF. T. G. BONNEY ON THE SO-CALLED ' G 



to considerable pressure and have undergone some micro-minera- 

 logical change. In both these groups the constituent minerals, as 

 a rule, exhibit a general similarity : the main difference consists in 

 their constitution and ordering. 



Thus, in these specimens of the ' Carboniferous gneiss ' from Gut- 

 tannen, we found, as usual, quartz, felspar (generally much 

 decomposed, but with both orthoclase and a plagioclase sometimes 

 recognizable), white mica (sometimes in fair-sized flakes, but 

 generally in crowded tiny films), biotite (often in well-developed 

 flakes, but occasionally in filmy aggregates or replaced by a greenish 

 secondary product), a dark mineral (sometimes an iron oxide, or 

 even pyrite, sometimes probably a carbonaceous mineral), together 

 with occasional rutile, zircon, epidote, apatite (?) and zoisite (?). 

 One specimen, at least, contains the remains of a garnet which has 

 been partly replaced by a green chloritic mineral. In short, we find 

 in these ' Carboniferous gneisses ' the same constituent minerals as 

 in the normal granitoid and gneissoid rocks of the Bernese Alps. 



I took two specimens from the block in which plant-remains had 

 been found ; each having a banded structure, but one being coarser 

 in texture than the other.^ Erom the coarser specimen I have had 

 two slices cut : one parallel with, the other transverse to the mineral 

 banding ; from the other specimen a single slice. All these, when 

 studied under the microscope, at the first glance, much resemble a 

 gneiss, but reveal, on a closer examination, a greater structural 

 variation than is usually found in a gneiss or a crystalline scliist. 

 At one moment a granitoid gneiss seems to occupy the field of the 

 microscope, at another a schistose aggregate of mica flakes, inter- 

 spersed with decomposed felspar and occasional quartzes. Although 

 a clastic structure is not as conspicuous as in an ordinary arkose or 

 greywacke, still the rock appears to be made up of fragments tightly 

 squeezed together ; the boundaries of these being indicated by darkish 

 lines, composed of minute minerals, such as mica, opacite, &c. In 

 one place a roundish constituent, in structure resembling a bit of 

 granite, is sharply appressed against a band composed of biotite and 

 decomposed felspar ; in another we find chiefly quartz and felspar 

 with hardly any biotite. The mica also differs from that usually 

 found in a crystalline rock which has been rendered fragmental by 

 crushing. It is not, as in such a case, sheared, tattered, and converted 

 into a string or ' cirrus ' of little flakes, but is simply crumpled and 

 pushed about. The felspars, also, though sometimes they pass by 

 crushing into a '• stringy ' aggregate of minute micaceous minerals, 

 as is so common among crushed crystallines, have often escaped 

 fairly well and are only replaced by microlithic decomposition- 

 products. Two other specimens, taken from rock in situ nearly 

 opposite Guttannen (one from near the path leading up from the 

 bridge), exhibit under the microscope more distinct indications of 

 an original clastic structure, and are, in my opinion, grits composed 

 of rather fine-grained (decomposed) detritus from a biotite-gneiss, 



^ One was from near the bottom of the block, the other from about a x&vd 

 above it. 



