343 



crystalline type. Rhyolitic (felsitic) tuffs are associated with the perlitic 



felsites at many points. 



I. II. III. 



Si0 2 ... 67-180 ... 69-044 ... 67'180 



A1 2 3 ... 16-650 ... 11-660 ... 16-650 



Fe 2 3 ... -559 ... 2-030 ... -559 



FeO ... 2-151 ... 1-449 ... 2-151 



CaO ... 2-352 ... 1-456 ... 2-352 



MgO ... 1-549 ... -540 ... 1-549 



K 2 ... 2-914 ... 3-570 ... 2-914 



Na 2 ... 4-032 ... 9-673 ... 4-082 



P 2 6 5 ... -179 ... tr. ... -179 



S0 3 ... tr. ... ... tr. 



C0 2 ... -885 ... tr. ... -885 



C ... -797 



H -752 -441 1-549 



100-000 99-863 100 OOO 



I. Armboth dyke. Analysis by HUGHES. WARD'S memoir on the northern part of the 



Lake District. 

 II. Augitic granophyre from summit of Carrock Fell. Analysis by HUGHES. Q.J.G.S., 



Vol. XXXII., p. 24. Also -137 of Sulphur. 

 III. Quartz-felsite. Threlkeld. Q.J.G.S., Vol. XXXII., p. 22. 



Cheviot District. Quartz-felsites are associated with the granite-mass 

 of the Cheviots. They occur as dykes in the surrounding volcanic series. 

 In colour they vary from pale red to dull purple. They are not con- 

 spicuously porphyritic, the individual crystals rarely exceeding a length 

 of 2 mm. Examined with a hand lens the most conspicuous mineral 

 is seen to be biotite which is evenly scattered through the rock 

 in the form of hexagonal tablets. Crystals of felspar, usually of the 

 same colour as the ground-mass, but occasionally weathering white, 

 may be recognized. Grains of quartz are also seen to be present, but 

 their importance as constituents of the rock is not recognized until the 

 thin sections are examined. Under the microscope the porphyritic 

 constituents are seen to present the usual characters. The boundaries 

 of the mica are intact. The quartz occurs in bipyramidal crystals and in 

 rounded or irregular grains. The ground-mass is micro- or crypto- 

 crystalline. Traces of granophyric structure may occasionally be 

 observed. Ill-defined specks, fibres and scales of ferrite are everywhere 

 present and lie superposed even in the thinnest sections. As a rule 

 these are irregularly scattered through the ground-mass, but in some 

 instances they give rise to small spherical aggregates. In one case the 

 ground-mass was seen to split up under crossed nicols into a coarse- 

 grained aggregate. The boundaries of the double refracting grains are 

 irregular and not recognizable in ordinary light. Each quartz-crystal 

 hi the rock formed the nucleus of one of these grains, so that the 

 space surrounding it extinguished simultaneously with the crystal. A 



