240 Correspondence — Rev. P)'of. Bonney. 



about the life of minerals, we may say that the flush of health, is 

 being replaced by the pallor of approaching death. 



4. As regards the twinning, which is no doubt a very strong 

 argument, I find a rather similar lamellar twinning (as it appears) 

 in hornblende (serpentineofMullion and Lower Pradanac). These seem 

 to be produced by the formation of a mineral (doubly refracting but 

 with diflferent extinction) along the cleavage planes parallel to oo p, 

 but I am by no means sure that this is the explanation of every 

 case. In my slide (of 1878) from the Rill it occurs in a mineral 

 which much more resembles a variety of enstatite than felspar. In 

 the slide cut from Mr. Teall's specimen the twinning both lamellar 

 and in two directions at high angles occurs in minerals which I 

 cannot distinguish from the low-tinted pyroxenes (I use the name 

 generically) in the other slides. Further, the twinning is produced 

 by narrow bands such as one obtains in the almost microlithio 

 felspars of lavas, not in those of holocrystalline rocks such as 

 picrites, gabbi'o, etc., and the outline of the grains is not that usual 

 in felspars, but curiously irregular. Moreover, I find in a slide cut 

 from the Carn Sparnack serpentine, both lamellar twinning and cross 

 twinning in the less highly altered part of the pyroxenic constituent. 

 I have measured the extinction angles of some of these compound 

 grains, but to discuss tlie result would unduly extend this letter, so 

 that I must content myself with affirming that these bear as much 

 resemblance to the twinning of a plagioclase felspar as those noted 

 by Mr. Teall, while they occur to such an extent that, if the mineral 

 were felspar, the rock could not fail to give macroscopic indications of 

 its presence. So, though I have endeavoured to approach the subject 

 with the ' open mind ' of some modern statesmen, I remain after 

 repeated examination of the question of ' the same opinion still ' 

 that the mineral is not felspar. My suggestion as to its name was 

 ' vague ' designedly, for two reasons : (a) that it is very often easier 

 to say what a mineral is not than what it is ; (6) that I am by no 

 means sure that these characteristics are exhibited by one mineral 

 only ; I believe it, however, to be always a member of the pyroxenic 

 group, A'iz. some variety of augite, hornblende, or enstatite. 



In conclusion, may I add that Mr. Teall appears to have slightly 

 misunderstood the drift of my remark quoted by Colonel McMahon. 

 Whether or not there is evidence of mechanical action on the serpentine 

 at Porthalla is hardly germane to the question. Of course I should 

 say that to assign the banded structure in this rock to pressure is at 

 present just as much an hypothesis as it is in regard to the banded 

 gabbro. But, apart from this, the difficulty, which I had felt and to 

 which Colonel McMahon referred, was this — that, when the gabbro 

 is so remarkably banded, then the serpentine shows little or no sign 

 of mechanical disturbance. Porthalla is some miles from both 

 Karakclews and the Landewednack district, and, so far as I know, 

 gabbro does not occur in association with serpentine either there or 

 near Mullion Cove. T. G. Bonney. 



