J. H. Teall — Cheviot Quartz-Fehites and Augite- Granites. 113 



SiOa 50-63J° 



AI2O3 -87 



FeiOa 3-33 



FeO 8-39 



MgO 13.01 



CaO 21-30 



NaoO 1-02 



K26 -50 



TiOa -79 



99-84 

 Sp. Gr. 3-372. 



It is therefore a non-aluminous iron-bearing augite of the diopside 

 t_ype allied to the augite of the augite-syenites. The tendency' of 

 the mineral to pass into serpentinous rather thanchloritic alteration 

 products is of course due to the absence of alumina. 



I now proceed to describe the rocks in which the above mineral 

 occurs. They are usually coarse-grained, holocrystalline, non- 

 porphyritic rocks, which vary considerably in aspect and in the 

 relative proportions of the different constituents. Some of the 

 varieties are dark and syenitic-looking ; others resemble a grey or 

 pink granite of normal aspect. Owing to the variability of the 

 rocks, it will be advisable to describe the special types which have 

 been examined. 



Linhope Burn, ^ mile above Linhope Farm. — A dark-coloured syenitic- 

 looking variety, composed of orthoclase, plagioclase, with low ex- 

 tinction angles, quartz, augite, biotite, magnetite, apatite and serpen- 

 tinous alteration products. Felspars of both kinds very abundant. 

 Augite well represented. Quartz in moderate quantity. 



Linhope Spout. — In the immediate neighbourhood of the waterfall 

 several very interesting rocks may be observed. The augite-granite 

 is here traversed by dykes and veins of quartz-felsite, which some- 

 times show under the microscope the micro-granitic, at other times 

 the granophyric structures. These two structures often co-exist in 

 the same slide, but as a rule one or other predominates, Biotite and 

 angite are occasionally present, but the latter mineral is decidedly 

 rare. I incline to the view that these are of the nature of con- 

 temporaneous veins, and if so, a very interesting conclusion, which 

 will be referred to later on, may be drawn from this fact. They are 

 evidently more acid in composition than the noi^mal rock. The 

 augite-granite of Linhope Spout is a coarse-grained rock, in which 

 flesh-coloured orthoclase, light-coloured plagioclase, biotite and a 

 dark green mineral (the augite) may be recognized by macro- 

 scopic examination. Under the microscope quartz is seen to occur 

 abundantly, the augite may be identified, and magnetite and apatite 

 may be recognized. One very interesting feature is the tendency 

 of the quartz and felspar to form micro-pegmatite. This is also 

 noticeable in the augite-granites of the Vosgps, and it may there- 

 fore be regarded as a characteristic feature of this class of rocks. 



Staindrop Rigg. — On the high ground between the Het Burn and 

 Staindrop Burn there are two conspicuous craggy bosses of a greyish 

 granite. These form Staindrop Eigg. A similar rock occurs low 



DECADE III. — VOL. II.— NO. III. 8 



