570 Correspondence — Miss M. B. Alder. 



hollows could only have done so on meeting with an obstruc- 

 tion such as a steep slope which would deflect the current of ice, 

 and make it acquire a gyratory motion which would enable it to 

 scoop out semicircularly backwards, and possibly at the same time 



downwards To be a cwm a hollow must be approximately 



curvilinear. Eain is doing all it can to destroy this curvilinearity. 

 Eain- streamlets in cwms are gullying their brims and channelling 

 their sides. A continuation of the process would render a cwm a 

 mere confluence of ravines. The chipping action of frost, aided by 

 rain, is tending to reduce the steepness of the encircling cliffs by 

 bevelling off their upper parts, and hiding their bases under screes. 

 Eain in a state of dispersion is possessed of so little power that it 

 cannot keep up a uniform abrasion of the sides of cwms so as to 



preserve their curvilinearity If a single stream cannot produce 



a cwm, several streams cannot combine so as to give rise to a cwm. 

 .... Springs would be incapable of undermining laterally so as to 

 leave a hollow at all approaching to the breadth of an average cwm, 

 while a spring undermining backwards would leave a ravine, not a 



cwm Springs and streams are the effects instead of the cause 



of cwms "What is the stream now doing in the upper part of 



its course, for instance under Glaslyn [Snowdon] ? Merely rutting a 

 continuous face of rock." The above are only a few quotations 

 selected from many passages to the same effect. I have likewise, in 

 articles in the Geol. Mag., etc., frequently referred to the evidences 

 furnished by glaciated rock-surfaces in peculiar positions, and by the 

 undisturbed curvilinearity of eskers, of the very small influence 

 exerted by rain and freshwater streams since the Glacial period. 

 While, however, agreeing with much that Mr. Goodchild has written, 

 I cannot help differing from him on many points — such, for instance, 

 as the forms he assigns to the traces of sea-action; but I fear I have 

 already trespassed too much on your increasingly valuable space. 



D. Mackintosh. 



" BOTTLEITE." 1 



Sir, — It gives me great pleasure to find that Mr. G. H. Kinahan 

 admits that the curious black mineral called " Bottleite," attached to 

 the base of some layers of granite, " seems due to crystalline struc- 

 ture, the substance being deposited from solution." (See his letter 

 Geol. Mag. for September last, p. 426.) As I have long held that 

 Flint is stalactitic, so I feel certain is Bottleite, a siliceous " stalactite" 

 which has dripped, so to speak, out of the granite. 



Whatever Bottleite and Flint are, Obsidian and Isopyre must be 

 classed with them. 2 More information is anxiously looked for by 

 Yours, etc. M. B. Alder. 



Fern Bank, Holywood, Cq. Down. 

 Sept. 22nd, 1875. 



1 Mr. Allport, F.G-.S., remarks: "'bottleite' and 'trachalite' are synonymous, 

 ' bottleite ' being tbe local name for a vitrioid rock pronounced to be 'trachalite.' " — 

 Edit. Geol. Mag. 



2 We venture to suggest tbat Miss Alder bas opened a wide field of inquiry for 

 Mr. Collins's proposed New Mineralogical Society. (See ante p. 569.) — Edit. 

 Geol. Mag. 



