142 MR. J. MOBRISON ON [vol. lxxiv> 



felsites to basic lamprophyres, covering a wider area and overlapping 

 the former in every direction. They are characterized by the 

 absence of porphyritic orthoclase, although quartz-grains are 

 commonly found even in some of the basic members. Last to be 

 intruded were the most basic lamprophyres, in corporal ing portions 

 of a felsitic magma. These show an increasing basicity, appar- 

 ently indicating an increasing divergence to more basic and more 

 acid poles. 



Two lines of variation are found: (1) horizontal, extreme acid 

 and basic types occurring in the outer parts of the area ; (2) direc- 

 tional, the earlier set coinciding in direction with the north- 

 north-west fractures transverse to the strike of the country-rock, 

 the later set with a general trend more or less at right angles 

 to the former, and corresponding approximately to the lines of 

 fracture to the east. 



The intrusions thus belong to two distinct periods, and appear 

 to be related to the two main series of crust-movements which 

 determine the geological structure of the area. 



In conclusion, I wish to record my thanks to the late Prof. T. 

 McKenny Hughes for granting facilities for the work at the 

 Sedgwick Museum, and for unrestricted access to the collection of 

 rocks and sections; also to all who have so kindlv and willingh" 

 rendered help in various ways. 



EXPLANATION OF PLATE XIII. 



Figs. 1 & 2 illustrate two stages of the mixed series, the rocks showing 

 a progressive increase in basicity. 



Fig. 1. Biotite-porphyrite. Hornblende (probably after angite) now appears, 

 and also forms a corrosion-rim round the quartz as seen on the 

 left. The dark mineral is mainly biotite, but a little hornblende 

 also is present. Sp. gr. = 2'6S4. X about 30 diameters. (See 

 p. 133.) T. 



2. Pilitic biotite-vogesite. On the upper margin is a rounded oligoclase- 



crystal, in the centre a pseudomorph mainly consisting of colourless 

 amphibole with some magnetite ; pale-green hornblende is seen on 

 the left associated with pyrrhotite and scattered throughout ; a 

 plate of biotite appears on the right. The felspar is principally 

 orthoclase. X about 25 diameters. (See p. 135.) M. 



3. Basic segregation in biotite-porphyrite. The minerals shown are 



sphene of irregular form, biotite uniformly dark, hornblende with 

 interior breaking away, and pyrrhotite ; the colourless mineral is 

 felspar, mainly plagioclase. X about 25 diameters. (See p. 132.) 



M. 



4. Intermediate rock of the Potter-Fell type. Crossed nicols. Consists 



essentially of felspar (much of it plagioclase) and biotite ; between 

 is microspherulitic orthoclase with a little quartz. X about 25 

 diameters. (See p. 125.) M. 



5. Spessartite. The rock is a plexus of laths of plagioclase-felspar and 



idiomorphic hornblende, with abundant magnetite, occasional calcite, 

 and some interstitial quartz. The two larger individuals are labra- 

 dorite. X about 25 diameters. (See p. 140.) M. 



