126 me. J. more [sox ox [vol. lxxiv, 



(b) the Mixed Series, each of which will be considered separately. 

 This division probably connotes a classification by age, a point 

 which will be considered later. 



(a) The Potter-Fell Type. 



The rocks of this group differ from those ahead}" described 

 chiefly in the diminished importance of quartz, but they also show 

 a considerable difference in microstructure. They are essentially 

 felspathic in constitution, consisting mainly of felspar with some 

 biotite. In the more acid members quartz may be present in the 

 form of small phenociysts, generally few in number, as well as in 

 the ground-mass. But it is always subsidiary in importance, and 

 in the more t}rpical representatives is restricted to the matrix, 

 where its role is interstitial. On the whole, therefore, the 

 affinities of these rocks lie between the syenites and 

 the diorites. 



The principal felspar is usually an oligoclase or oligoclase- 

 andesine, and the amount of biotite is variable. The gradual 

 decrease in quartz and the increase in biotite mark one line of 

 advance to the more basic derivatives of the magma. Another 

 line of development will be considered later ; but the present 

 marks the normal change. 



The intrusions are not numerous, yet their distribution may 

 have some significance. Certain well-marked features serve to 

 distinguish them from all others. The rocks, pale to dull red 

 in colour, are generally porphyritic ; but the phenociysts are of 

 a small order, consisting as a rule of little felspar-prisms. Well- 

 jointed parallel to the boundaries, they split readily into roughly- 

 parallel slabs, the irregular sides of which are coated with a greenish 

 aggregate of chlorite and mica. Differential weathering produces 

 a marked longitudinal fluting. 



The Potter-Fell intrusion, which is taken as the type, may be 

 traced at intervals across the fell for a mile or so in an east-north- 

 easterly direction, forming a fairly-regular sill intruded into the 

 Kirkby-Moor Flags. Its outcrop on the eastern flank is abruptly 

 shifted to the south-south-east, reappearing some distance away 

 with the same trend as before. A further deviation subsequently 

 occurs. Specimens from different parts are very similar, and all 

 are evidently part of the same intrusion. The rock contains small 

 porphyritic felspars often striated, greenish needles, and a little 

 mica in a dull-red felspathic matrix. Microscopically the ground - 

 mass is composed of small idiomorphic felspars ; oligoclase is 

 always very abundant, sometimes in excess, and a variable amount 

 of spherulitic material (mainly orthoclase) occurs interstitially. 

 The principal phenocryst is oligoclase-andesine, often zoned ; but 

 andesine appears in some sections, chiefly marginal, while orthoclase 

 is always subordinate. Biotite is abundant, though it is not yet so 

 important as in the lamprophyres. The rock is akin to the porphy- 

 rites in many respects, and may be termed a biotite-porphyrite. 



