OF THE SOUTH OP THE ISLE OF MAN. 449 



at Rockraonnt. . . .about the exact cliaracter of which there seems to 

 be some little doubt," and of which Mr. Kutley says, " The consti- 

 tuents are labradorite, serpentine ])seudomorf)hous after olivine, and 

 possibly in some cases after pyroxene, ilraenite, magnetite, and some 

 calcite." The specific gravity, 2-2fi, quoted for the specimen 

 analysed is erroneous ; it should bo 2*70. The hand-specimen does 

 not agree very well with that collected by me. The silica per- 

 centage agrees closely with that of the augite-porphyrite (termed 

 by Mr. llutley "altered basalt") of Scarlet Stack, but the per- 

 centage of lime and magnesia is much higher. 



Til. Summary. 



Omitting from consideration the Foxdale granite, the oldest 

 igneous rocks of the south of the Isle of Man appear to be the Dia- 

 base series which occurs as dykes with a prevalent north-easterly 

 strike, intrusive in the Lower-Silurian slates. These dykes, best 

 seen at Langness, are of pre-Carboniferous age, as they do not 

 penetrate the Carboniferous basement-conglomerate. 



The micro-granite dyke at Crosby is interesting from the parallel 

 structure of its salbands. It is intrusive in the Lower-Silurian 

 slates and is probably newer than the Poxdale granite*, which 

 appears to be post-Lower Silurian, since it has altered the slates and 

 thrown them into an anticlinal, and pre-Carboniferous, since the 

 Carboniferous conglomerate overlies the slates almost horizontally 

 at Langness. 



AVe next meet with volcanic rocks of Lower-Carboniferous age. 

 This is the augite-porphyrite series, and consists of tuff, breccia, ag- 

 glomerate, bedded lava, and intrusive masses, exposed in a narrow 

 strip a mile and a quarter long, extending from Poolvash (Poyll 

 Yaaish) to Scarlet Point. The succession of events appears to have 

 been as follows : — During, or after, the deposit of the Poolvash 

 limestone a vent was opened from which fine volcanic ashes were 

 ejected and fell into the sea, forming bedded tuff. At intervals 

 between the eruptions the black, so-called Poolvash marble was de- 

 posited, and thus came to be interstratified with the tuff. Probably 

 the vent became plugged up, and the violent eruption accompanying 

 the blowing up of the plug provided the material for the agglomerate 

 which overlies the tuff near Scarlet Point. Then lava welled forth 

 and overspread the agglomerate. Finally the volcano became extinct, 

 and by denudation the intrusive mass of the Stack, which I regard 

 as a volcanic neck, was exposed. It was probably at the close of 

 the volcanic activity that a melaphyre dyke containing porphyritic 

 plagioclase, augite, and pseudomorphs after olivine, and resembling 

 the porphyritic olivine-basalt of the Lion's Haunch, Edinburgh, was 

 formed. 



At Poortown, north-west of St, John's, there occurs an intrusive 



* See Cumming's 'Isle of Man/ p. 175. 



