Prof. T. G. Bonney — The Enstatitic Lama of Eiicott Hill. 79 



I may remark, contains some sections of the supposed altered 

 enstatite. Many of these exhibit one well-marked and fairly frequent 

 cleavage, with two others much less perfectly developed ; one being 

 ■yery nearly at right angles to, and the other making an angle of about 

 26° or 27° with the first. Thus these two include between them an 

 angle not far from G0°, but where the principal cleavage is not 

 visible in the slice, then the irregular cracks appear to meet much 

 more nearly at right angles. The slide from the normal rock contains 

 five or six crystalline grains, roughly clustered together. Not one 

 has quite escaped change, but in several we see a rather broad fibrous- 

 looking border of a pale green colour, which gives uniform and clear 

 tints with the crossed Nicols, while the interior is confusedly fibrous 

 in structui'e and in large part dark, besides being a rather stronger 

 green colour with ordinary light. Here and there we have a graia 

 which is not much more altered than the one already described. 



Above the porphyritic lava just described comes a series of lavas 

 noticed in Mr. Ward's paper. I possess a specimen from the lowest 

 of these (I believe that numbered 6 in his section). It is porphyritic, 

 but the felspar crystals are much smaller than in the other rock, not 

 generally exceeding a quarter of an inch in the longer diameter. 

 These under the microscope appear to be more broken and corroded 

 than they are in the rock below. The altered " enstatite " is also 

 present in about the same quantity, but it too has a slightly more 

 corroded aspect and is more thickly black-bordered than in the other. 

 The ground-mass is of the same general character, but the crystallites 

 are smaller, and the base, which with a low power is darker, seems 

 to be of a green hue, and more crowded with microlitlis. The dark 

 border of the enstatite also seems to be resolved into minute granules 

 of a partly translucent mineral, probably augite, mingled with ferrite. 



In the lowest of the lavas at Falcon Crag I find a little of what I 

 believe to be the altered enstatite, though it is much more irregular 

 in form, more interrupted by inclusions, and altogether less well 

 characterized than at Eycott Hill, so that I should hardly have 

 ventured on identifying it from these slides. In this lava there is 

 also a fair quantity of well-preserved and chai'acteristic augite 

 (diameter commonly about '03 to "Oo inch). I do not detect the 

 enstatite in either of the slides from the next two lava-flows. 



I think then there can be no doubt that the Eycott Hill lavas 

 contain a variety of enstatite. It might be possible, but it would 

 be difficult, to isolate a sufficient quantity for chemical analysis, but 

 to my mind the evidence as to its nature is already sufficiently clear, 

 and the only point on which we require enlightenment is whether 

 enstatite or bronzite would be the better name. The settlement of 

 this point I willingly leave to any one who thinks it worth the time 

 and labour, for I feel confident that the mineral is a bisilicate of 

 magnesia with some iron. 



From the aspect of the ground-mass of the Eycott Hill rock, and 

 the presence of the bisilicate, enstatite, instead of the unisilicate, 

 olivine, I should not have been surprised if it had proved to be 

 chemically more nearly related to the Falcon Crag lavas than to those 



