434 Mil. n. uoBsoN on tue igneous bocks 



the tiiif can be shown to be of the age of the Carboniferous Poolvash 

 black marble, which is regarded by Mr. E,. Etheridge, jun., as pro- 

 bably referable to the horizon of the Upper Limestone shales *. The 

 whole of the augite-porphyrite series probably belongs to the same 

 period, as will be shown subsequently. 



(d) The Melapli^jre Bijhe. — This dyke intervenes between the 

 Lower Limestones and the augite-porphyrite agglomerate at Scarlet 

 Point, and strikes N. 61° W., apparently occupying a line of fault. 



It resembles the augite-porphyrites in microscopic structure, the 

 only important difference being the occurrence in it of porphyritic 

 augite t- Its specific gravity, 2*77 to 2*8] , agrees very fairly with 

 that of the augite-porphyrite of the Stack [522], which is 2*78. 

 Hence I regard it as a final product of the series of augite-porphyrite 

 eruptions, and therefore as probably of Lower Carboniferous age. 



(e) Tlie Picrite-jiorjphyrite. — This occurs in the village of Poortown, 

 near Eockmount, north-west of St. Johns. It is quarried for road- 

 metal. Cumming terms this variously " hornblende-rock " (he has 

 probably mistaken the augite for hornblende) and " porphyry." 

 He states that the same rock occurs at Cronk Urley and at Port St. 

 Mary, and regards it as post-Carboniferous in age, " for it appears to 

 have tilted . . the old red sandstone [Carboniferous] .... to a high 

 angle " J, and the same rock is " developed almost continuously along 

 the fault .... running from Perwick Bay through Port St. Mary, 

 Strandhall, and Athol Bridge cutting off at once all the car- 

 boniferous series to the N.W. of this line." Henslow § also mentions 

 this rock as occurring at Port St. Mary in these words : " A bed of 

 greenstone, in which the hornblende is remarkably well crystallized, 

 occurs to the south of Port le Murray .... forming a ridge eight or 

 ten feet broad." Although I have not seen this dyke I believe 

 Berger and Cumming to be quite correct, for in September J 887 

 I observed on the shore on the inner side of the pier a large block 

 of dark green rock with the drill-holes in it, evidently blasted in 

 making the new pier at Port St. Mary, and it agrees with the 

 Poortown rock, when examined microscopically, in containing similar 

 porphyritic augite-crystals, though they are rather less plentiful. 

 Probably the pier is built upon the dyke. 



(f) The OUvine-dole7'ite Series. — This is exposed in the form of 

 dykes between tide-marks on the east shore of Bay-ny-Carrickey from 

 Kentraugh to Scarlet Point ; on both the west and the east shores 

 of Castletown Bay, and also on the east side of Langness ; at Derby- 

 haven breakwater and between Eonaldsway and Cass-ny-Hawin. 

 These dykes are from 8 inches or less to 30 feet wide, and the 

 average strike calculated from 36 observations is N. 44° W. With 

 one exception striking Ts". 26° E., the strike of them all is comprised 



* Etheridge in Home's paper, Trans. Edin. Geol. Soc. vol. ii. (1874) p. 331. 



t Throughout this paper I use the term 'porphyritic' in the sense of 

 Rosenbusch, *Mikr. Physiogr. d. massig. Gest.' (1887) pp. 339, 340, i. e. irre- 

 spective of size, for the first (intratelluric) generation of crystals in a rock in 

 Avhich there has been recurrence of phase. 



I ' Isle of Man,' pp. 194, 245, and note on p. 239. 



§ Trans. Geol. Soc. vol. v. (1821) p. 497. 



