MINEHALOGICAL NOTES : NO. IV. ANDERSON. 267 



has recently given me additional information regarding the 

 locality. The best specimens in the collection were obtained some 

 thirty-five years ago by the late Mr. Thomas Clarke of Oban 

 while blasting up a side channel in the granite in order to divert 

 the stream from the bed of Oban Creek and facilitate the search 

 for gold and tin-ore therein. Detached crystals of felspar, accom- 

 panied by tourmaline, topaz, cassiterite and gold are sometimes 

 found in the alluv'ial wash in the neighbourhood of Oban. 



The granite of Oban belongs to the " later and more acid type " 

 of Andrews'* ; associated with it is a series of eurites, often 

 pegmatitic, and it is mainly, if not entirely, from the pegmatite 

 phases that the large crystals of orthoclase here described have 

 come. A common characteristic in hand specimens is a graphic 

 intergrowth of quartz and felspar ; this is seen on a large scale 

 in the specimen shown in PI. 1., fig. 1, where the several 

 quartz crystals project from the felspar with their vertical axes 

 parallel. One interesting specimen consists of a group of large 

 crystals of smoky quartz, the core of one being an elongated 

 crystal of felspar twinned on the Baveno law but without ter- 

 minations. A somewhat similar association is seen in PI. li., 

 where a well-developed Baveno twin is partly embedded in a 

 smoky quartz crystal. Besides quartz (usually smoky), which is 

 a constant companion of the orthoclase, we find associated with 

 it, tourmaline, in the usual striated columnar crystals, and a 

 plagioclase felspar, which, from refractive index and extinction 

 angles, is found to be near albite. Plate Hi., is a photograph 

 of a slab consisting of large, buff, orthoclase crystals, much 

 decomposed, seated on which are fresher, whitish crystals of 

 albite in intercrossing pei'icline twins. A fine example of a 

 Baveno doublet is shown in PI. xlviii., fig. 2 ; it has the usual 

 habit elongated parallel to the axis [c, 6]. The two portions are 

 not quite symmetrical to the combination plane, the face c' (001) 

 slightly overlapping the face b (010), with which it is practically 

 coplanar, but the boundaries of the two segments are easily trace- 

 able by the aid of the series of more or less parallel markings 

 present on every face and having a direction on each approxi- 

 mately parallel to the intersection of the particular face with the 

 plane of the pinacoid (100). These lines of corrosion are some- 

 what less pi'onounced on the prism m (110), which still retains a 

 dimly vitreous lusti'e. This crystal measures about 4x2 cm. 



A more complicated twin is represented in PI. xlviii., fig. 3. It 

 may be interpreted either as a triplet according to the Baveno 



^ Andrews— Kec. Geol. Survey N. S. Wales, viii., 2, 1905, p. 116. 



