280 



resemble, for example, the inclusions in the felspar of the Baltimore gabbro 

 described and figured by Mr. WILLIAMS.^ Sections at right angles to the 

 twinning planes are often seen under ordinary light to be finely striated in con- 

 sequence of the arrangement of the inclusions in lines or rather planes parallel 

 to the brachy-pinacoid. Under polarized light the same sections show the 

 characteristic banding of plagioclase, and the maximum angle which has 

 been observed between the extinction positions of adjacent lamellae is 34 

 (maximum for oligoclase 37). The pyroxene occurs in good-sized grains 

 having the cleavages and extinction angles of augite and also as minute 

 crystalline granules in the ground-mass. The larger grains rarely show any 

 traces of crystalline form but they are frequently twinned. In the absence 

 of colour and in general aspect they resemble the augite of the augite- 

 diorites and augite-granites of the district. A pale brown mica occurs in 

 minute scales. It is frequently aggregated in patches and when this is the 

 case several scales overlap in the thickness of the preparation. This mica 

 closely resembles that developed in the dolerites of Cornwall near their 

 contact with granite. (2) Apatite occurs abundantly. Magnetite is present 

 in irregular grains of considerable size and also as minute granules. The 

 ground-mass consists of an ultimate base of clear isotropic glass thickly 

 crowded with opaque and reddish brown granules, minute crystalline grains 

 of pyroxene and, in places, with the mica above referred to. 



Rocks unaffected by surface alteration form only a small portion of the 

 Cheviot district. The dominant rocks are lavas and tuffs which must origin- 

 ally have been similar in composition and texture to modern andesites, but 

 which are now in a more or less altered condition. The term porphyrite 

 may be conveniently applied to these rocks but it must be distinctly 

 remembered that the term has reference merely to secondary characters. 

 It is simply equivalent to the expression altered andesite. 



Now the Cheviot porphyrites differ from the andesites in having their 

 felspars more or less decomposed, their ferro-magnesian minerals replaced 

 by bastite, chlorite or other green decomposition products and also in 

 frequently containing a certain amount of disseminated ferric oxide (ferrite). 

 Freshly fractured surfaces are always dull (never resinous or vitreous) and 

 of a purplish, reddish or dark bluish colour. The colour is dependent 

 upon the state of oxidation of the iron-ores scattered through the mass. 

 Two types of porphyrite have been recognized pyroxene-porphyrite and 

 mica-porphyrite. 



A very typical pyroxene-porphyrite (the term pyroxene is used to 

 cover both the monoclinic and rhombic minerals) is represented in Fig. 2, 

 Plate XXXYII. This rock was originally a labradorite-enstatite-andesite. 

 The ground-mass is a typical " mikrolithenfilz." Excellent examples of 

 pyroxene-porphyrite may be obtained from almost any portion of the 

 district occupied by volcanic rocks. 



(1) Bull. U. S. Geol. Survey, No. 28. 



(2) As a large mass of granite occurs near the exposure of the above rock it is quite 

 possible that the peculiarity in the felspar and the presence of the brown mica may be features 

 due to contact metamorphism. 



