Prof. T. O. Bonneij—The Emtatitic Lavas of Etjcoti Bill 77 



The felspar crystals, with the above described greenish spots, and 

 some grains of iron oxide, belong to an earlier stage of consolidation, 

 or, possibly one should say, of crystal-building. As Mr. Ward points 

 out, they cannot have been separated out of the mass when it flowed 

 as a lava. I have little to add to his careful description, except that 

 I believe these felspars to be labradorite, and doubt the possibility 

 of any of them being orthoclase. In Mr. Postletliwaite's specimen 

 a few scales of iron-glance are scattered about in the felspar crystals, 

 but not in such numbers as one would expect from their ruddy tint 

 before being sliced. The soft dark spots are regarded by Mr. Ward 

 as pseudomorphs in many cases after augite, and some, he says, 

 "seem to be after olivine, presenting the form and much-iissured 

 appearance of that mineral. I have detected grains of olivine in an 

 unaltered condition in some of these laA^as, and therefore think there 

 can be no doubt that both it and augite were common constituents 

 at one time, though both have been so much replaced by pseudo- 

 morphous minerals through subsequent alteration." It is of course 

 quite possible that my lamented friend may have detected olivine 

 in some of the upper lavas, which I have not examined ; but I think 

 he has erred in his reference of these dark green spots either to augite 

 or olivine, as will be seen from the following description. But I do 

 not in the least doubt the presence of moderate-sized crystals of 

 augite, which however seems to be always in fair if not good pre- 

 servation, in addition to that which occurs in the ground-mass. 



The mineral of which we speak is obviously a magnesian silicate, 

 which has generally been converted, since the consolidation of the 

 rock, into a kind of serpentine. In form it is usually a rather 

 irregular oblong, about -twice as long as it is broad, with a slightly 

 worn or corroded aspect at its edges ; and it is very commonly 

 bordered by clustered opacite, as is frequent with hornblende and 

 mica in anandesite. Hence the external crystalline form is generally 

 not well defined. There is one well-marked cleavage parallel with 

 the longer sides of the grain, with, I think, an indication of a second, 

 which makes a large, if not a right angle with the other, and possibly 

 a third ; but these are often only suggested by the presence of rather 

 irregular cracks, so that it is not possible to be certain about them. 

 Indications of a fibrous structure, parallel with the dominant cleavage, 

 may also be observed. With transmitted light the usual colour is 

 a pale sap-green, but sometimes the inner, sometimes the outer part 

 of the crystal is slightly browner. The mineral has evidently under- 

 gone alteration, but in one ci-ystal a portion remains apparently 

 unchanged, and this is of a pale greenish straw colour. All exhibit 

 a fairly well-marked dichroism. When the principal cleavage lines 

 are parallel with the vibration plane of the lower Nicol, the tint is a 

 pale sap-green ; when perpendicular, it is a pale yellowish-green. 

 With crossed Nicols, extinction takes place when the dominant 

 cleavage lines are parallel with either vibration plane; when they 

 are at an angle of 45° with these, the colour in the unaltered part is 

 a straw-yellow, the tints in the altered parts being paler, and such 

 as are usual with serpentinous minerals. I notice that the browner 



