10 Rev. 0. Fisher — On Cirques and Taluses. 



sist partly of plagioclase. It is known that some crystals are formed of alternating 

 laminaj ojf triclinic and monoclinic felspar. 



Fig. 6. — Crystal of triclinic felspar, in the porphyritic pitchstone, from Goatfell 

 X 5 diameters. The interior is full of cavities containing brown glass. See p. 7. 



Fig. 7. — The same in polarized light. 



Figs. 8, 9, 10. — Imperfect quartz crystals, containing glass cavities and deep 

 indentations filled with the glassy base. See p. 4. 



Fig. 11. — Minute glass cavity of characteristic crystalline form, magnified 180 

 diameters. It contains a gas cavity and also a portion of the base. See p. 4. 



Fig. 12. — One side of an imperfect quartz crystal, showing mode of formation 

 of the glass cavities (p. 4). In the original drawing the sides of the neck are rather 

 closer, and the lines curved instead of straight. 



Fig. 13. — Group of crystals, surrounded by streams of belonites. Section of a 

 porphyritic pitchstone from a dyke near King's Cove. See p. 5. 



II. — On Cirques anb Taluses. 

 By the Rev. 0. Fisher, M.A., F.G.S. 



I HAVE had the pleasure of reading Mr. Bonney's paper, " On a 

 Cirque at Skye," and send you these few lines rather as an ap- 

 pendage to it than in the spirit of criticism. I think Mr. Bonney has 

 made out the greater part of his case very well ; but being a rather 

 more "ardent glacialist" than he (though far less acquainted with 

 glaciers), I do not think he has attributed quite enough to their 

 operation in the formation of a cirque. Speaking of a Glacial 

 period, he says,^ " The cliffs would still be cut back, by water in 

 summer, by frost in winter ; the talus borne away or crushed by the 

 glacier, the rocks below somewhat worn and rounded, but still the 

 completeness of the Cirque as a whole forbids us — unless we assign 

 it entirely to glacial action — to suppose that it was more than 

 slightly altered by this." 



I think that the work cursorily attributed to the glacier in this 

 summary is of the essence of the question, and that without the 

 glacier the cirque would not have been formed. I reason thus : If a 

 homogeneous rock is subjected to simple weathering, the result is, 

 that the surface scales off to an equal depth all over during every 

 successive season. For instance, the vertical face of a deserted 

 chalk pit continues vertical in the upper part until the whole face 

 becomes masked by a talus which grows upwards from the base. 

 I have shown in a former number of this Magazine ^ that behind 

 this talus a parabolic contour must be formed. But what is enough 

 for the present purpose is, that above the talus the face continues 

 vertical. If then by any means the talus should be removed as fast 

 as it is produced, the vertical form will be perpetually maintained, 

 and carried farther back into the hill every year. The same, no 

 doubt, will happen with any rock which is homogeneous, and suffi- 

 ciently firm to stand in a vertical face If it be affected by vertical 

 joints, its destruction will be more rapid. 



Now I conceive that this is what has happened in these cirques, 

 and that the glaciers which formerly occupied them have been the 

 carriers which removed the talus. Their former action as such is 



1 Geol. Mag., Vol. VIII., p. 539. ^ Geol. Mag., Vol. III., p. 354. 



