OF THE SOUTH OF THE ISLE OF MAN. 441 



Tho porphyritic constituents resemble those of 205 a, but quartz 

 was also observed. They vary in diameter from '37 millim. to 

 1*57 millim. The boundary between the dyke and the micaceous 

 schist is perfectly sharply defined, thoui^h small intrusive tonj^ues of 

 micro-granite penetrate the schist. In the schist the brown mica 

 is arranged at quite oblique angles to the foliation planes. This is 

 seen even in the hand-specimen. 



I hesitate as to whether I should ascribe the foliation of the 

 salbauds of this dyke to dynamic metamorphism, but am inclined to 

 do so. 



(c) The Aurfite-porphiirite Series. — In the following description 

 the numbers refer to the localities on the map of Scarlet Point 

 (PI. XIY.). 



The macroscopic characters differ somewhat according to the more 

 or less completely crystalline state of the rock. The rock of the 

 Stack [522] is most crystalline, and is of a grey colour with a tinge 

 of green. Porphyritic plagioclase-crystals, as much as i inch long, 

 are embedded in a groundmass of which the constituents are not 

 determinable macroscopically. A specimen of 500 a is similar, 

 except that the groundmass is more completely compact, and rarely 

 pseudomorphs after porphyritic olivine occur. In the breccia [200j 

 and similar very amygdaloidal and less crystalline specimens, no por- 

 phyritic plagioclase can be made out, and the rock often looks very 

 like a piece of light^coloured Carboniferous limestone. 



All these rocks weather reddish-brown at tho surface where not 

 exposed to the sea, but dark brown where exposed to it at the Stack, 

 &c. Where weathering has removed the amygdules they are often 

 vesicular. Internally the weathered parts are greenish. 



Messrs. Dickson and Holland * give the analysis of a specimen 

 from the summit of Scarlet Stack, showing a silica percentage of 

 46*70. The specific gravity, 2'62, quoted for the specimen is in- 

 correct ; I have redetermined it from Mr. Dickson's specimen to 

 be 2'76. The average specific gravity of the augite-porphyrite is as 

 follows : — 



Specific gravity of 



No. 156 =2-75 155 = 2-76 



„ 500a=2-75 522 = 2-78. 



Average specific gravity 2-76. 



I will now describe a section cut for the microscope from the lava- 

 bed [155], which I submitted to Prof. Posenbusch, who pronounced 

 it to be an augite-porphyrite allied to the spilite t type. 



The structure is distinctly porphyritic, the porphyritic constituents 

 being large idiomorphic plagioclase-crystals. The groundmass con- 

 sists of much smaller lath-shaped plagioclase-crystals, black iron-ore, 

 chlorite, titanite, and some calcareous alteration-products. 



The porphyritic plagioclase-crystals are of large size and distinctly 

 idiomorphic. One of average size measured 2-81 millim. x '71 miUim. 

 Mostly polysynthetic twins, some of them show twinning on the 



* Proc. L'pool Geol. Soc. vol. vi. pt. i. (1889) p. 128. 



t Kosenbusch, ' Mikr. Physiogr. d. massig. Gest.' (1887) p. 493. 



Q.J.G.S. No. 187. 2h 



