272 



affinities with the rocks described as quartz-gabbros (augite-diorites) from 

 Carrock Fell already referred to and with some of the augite-granites to 

 the Cheviot District, which we now proceed to describe. 



The Cheviot District is characterized by the extensive development of 

 lavas and tufts of intermediate composition andesites and porphyrites. 

 In the central portion of the district, about Hedgehope and Staindrop, we 

 find granitic rocks which doubtless belong to the same phase of igneous 

 activity as the lavas and tuffs. These granitic rocks vary considerably in 

 the relative proportions of the different constituents ; some are dark in 

 colour owing to the abundance of ferro-magnesian constituents, others are 

 grey or pink and similar in general aspect to many normal granites. The 

 dark-coloured rocks closely resemble in general appearance the so-called 

 syenite of Markfield and Groby. They may be studied in the neighbour- 

 hood of Linhope Spout. Microscopic examination merely serves to confirm 

 the impressions derived from an examination of the hand specimens. The 

 rocks consist of idiomorphic plagioclase, augite, biotite, magnetite or 

 ilmenite and a micro-pegmatitic (grnnophyric) matrix. The difference 

 between these rocks and those of Charnwood lies in the absence of original 

 hornblende and the better preservation of the augite and biotite. Epidote, 

 so abundantly developed in the Charnwood rocks, has not been observed in 

 the corresponding rocks from the Cheviot district. 



The augite of these rocks is present in the form of irregular grains, 

 granular aggregates and sometimes, though rarely, in more or less definite 

 crystals. It is almost colourless in thin section. Inclusions of magnetite 

 are not uncommon. Transverse sections show the characteristic cleavages, 

 and longitudinal sections give a maximum extinction of 43 or 44. Twin- 

 ning may occasionally be observed. Dr. ALPHONSE MEKIAN (X. J. Beilage, 

 Band. Ill, p. 262) has isolated and analysed the augite of an allied rock 

 from Laveline in the Yosges. It possessed the following composition : 



8i0 2 ... 50-63 



Al,0, ... -87 



Fe.,6.. ... 3-33 



FeO ... 8-30 



MgO. ... 13-01 



CaO ... 21-30 



Na.,0 ... 1-02 



K.,6 ... .50 



TiO -79 



99*4 



Sp. Gr. ... 3-372 



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

 (malacolite. Rosenbusch ). 



The lighter coloured and more typical granites from Staindrop, Combe 

 Fell and Hedgehope are closely related to the rocks just described. They 

 will be referred to in the chapter on granites. 



The interesting group of the enstatite-diorites is represented in this 

 country by the rock so extensively quarried in the neighbourhood of 

 Penmaenmawr, Caernarvonshire. This rock has been described by 



