Dr. F. M. Stapff — Sand- grains in Micaceous Gneiss. 155 



rather than a plane parallelism ; but recently-exposed large faces of 

 the rock allowed a more satisfactory decision to be arrived at. 



As some 70 examples (belonging to as many St. Gothard Col- 

 lections) of the rock in question are preserved in different Museums, 

 it would not be difficult for anyone to examine its petrographical 

 details. I give here (Fig. 2, natural size) an autotypio print of &, 



^' 



Fig. 2. — Natural size. Micaceous gneiss with sand-grains. Bed 130 (north) 

 St. Gothard Tunnel. 



polished face, which has recently been cut from a specimen in the 

 direction of its most visible " parallel structure," as indicated by 

 the mica ; but some wavy undulating bands on the design indicate 

 another orientation, which almost agrees with the trend of the im- 

 bedded quartz-grains. The quartz-grains (white spots on drawing) 

 are, by their shape and lustre, easily recognized and distinguished 

 from the felspar grains, and I cannot give a better description of 

 their form, size, number and distribution, than can be seen in the 

 figure. 



The autotypio figures 3 and 4, which are taken by micro- 

 photogi'aphy from my original thin slices (described in the 

 '• Geologische Durchschnitte und Tabellen " beforenamed) reproduce 

 respectively a grain of quartz and a grain of felspar magnified 

 16-17 times. In order to show the marginal lines between the 

 groundmass and the imbedded grains as clearly as possible, I have 

 taken the photograph by polarized light ; but by this method other 

 particulars have been obscured or are not shown. 



The quartz of the grains is apparently different from the constituent 

 quartz of the enveloping micaceous gneiss. The former agrees with 

 the quartz crystals occuri-ing in porphyries, the latter with the quartz 

 in granites and gneisses. Each quartz-grain of our rock forms a 

 single crystallographic individual, which is frequently split, 

 in one direction by a few clear parallel fissures, and in other 

 directions by single irregular cracks. Sometimes the fragments of 

 the fractured grain have not been at all dislocated, and then the 

 polarization colour is homogeneous throughout the whole grain ; in 



