Correspondence — Rev. Prof. Bonney. 239 



respect genuine. I have often thought of publishing these facts, 

 and I think I shall do so ere long." 



What became of Dr. Bowerbank's collections ? 



I am quite aware that, as a rule, a geologist will not trust any one 

 to observe correctly except himself. But on questions of this kind, 

 where the evidence is destroyed in the process of being obtained, 

 autopsy is impossible. We are therefore obliged to rely upon 

 cumulative evidence, the weight of which depends upon the circum- 

 stance, that it is highly improbable that every observation should be 

 erroneous, while at the same time a single correct one is sufficient to 

 prove the point at issue. 0. Fisher. 



Harlton, Cambridge, 4iA April, 



FELSPAR IN THE LIZARD SERPENTmE. 



Sir, — May I be permitted to state, as briefly as possible, the 

 reasons why the characters described by my friend Mr. Teall in his 

 letter on "The Lizard Serpentines," fail to convince me that the 

 mineral in question, which occurs in the Eill rock, is really felspar? 

 As he rightly says, the identification of a mineral under the micro- 

 scope is often more or less a matter of inference. Hence it is 

 occasionally quite possible for two observers, both of some experience, 

 to take different views. I do not then attribute a mistake to him in 

 the ordinary sense of the word, nor wish in any degree to detract 

 from the value of his work. The point is one of considerable 

 interest, where there is ample room for two opinions. 



To prevent any misunderstanding, let me say that I do not in the 

 least deny that felspar may occur as an accidental constituent in a 

 peridotite, and, if it occurred anywhere in the Lizard Serpentines, I 

 should expect it, as will be seen from ray remarks on that of Gue Graze, 

 in the serpentine of the Kynance-Mullion district.' The difficulty of 

 determining this particular mineral is not a new one to me, as I had 

 to consider it nine years ago when preparing the above-named paper. 



The following are my reasons, so far as they can be expressed on 

 paper : — 



1. The texture and aspect of the mineral in question, seen under 

 the microscope, do not appear to me exactly identical with those of a 

 felspar, but remind me rather of a pyroxenic mineral. 



2. The brown earthy decomposition of the mineral seems to differ 

 slightly from that of a felspar, and I find a similar decomposition in 

 some grains of decomposing hornblende (mineral identified by 

 cleavage and extinction) in the serpentine of Lower Pradanac, also 

 in that of MuUion and Helston Road. I have also seen a similar 

 decomposition in bastite or enstatite. 



3. As to the tints seen between crossed Nicols. Low neuti'al tints 

 are not rare in enstatites. I have noted them in augites, when some- 

 what decomposed, and in certain hornblendes. In my slides from 

 Lower Pradanac the hornblende generally shows chromatic polar- 

 ization, but some grains exhibit these low neutral tints. I believe it 

 indicates incipient decomposition. As we have lately heard much 



1 Quart. Journ. Geol. Soc. yoL xxxiii. p. 918. 



