17G 



THE GEOLOGIST. 



N.E. of Castletown, where the Old Bed is seen resting unconform- 

 ablj upon the slaty schists as a conglomerate of quartz and Silurian 

 pebbles, enclosed 'first in a deep red clayey matrix, then in a grey 

 calcareous one, and then passing conformably into the dark lime- 

 stone above. 



Eesting conformably upon the Old Eed conglomerate are the lower 

 beds of the Carboniferous limestone, and with such regularity does 

 the transition take place, that the characteristic fossils of the one 

 formation are found mingled with those of the other. The Carboni- 

 ferous limestone is now found only in the south of the island, but it 

 formerly existed in the north, near Peel ; and there is evidence of its 

 continuance below low-water mark off the Peel coast. 



In the south it covers an area of about twelve square miles, com- 

 prehending the whole of the S.E, corner of the island from Coshna- 

 hawin, near the mouth of the Santonsburn, round the coast to Ber- 

 wick Bay, a little south of Port 8t. Mary, thence along a line of fault 

 inland to Athol Bridge, and from Athol Bridge to Coshnahawin Head, 

 with the exception of the southern extremity of Langness. Through- 

 out the whole of this extensive area the limestone immediately under- 

 lies the superficial covering of Drift clay, etc., and in this locality, 

 particularly along the coast from Coshnahawin Head to Port St. 

 Mary, the phenomena of limestone deposits may be studied with 

 great advrntage. 



The difierences of composition, texture, and colour among the 

 different limestone beds of this area are very great. Thus at Derby- 

 haven and Port St. Mary on the coast, and at Ballasalla in the inte- 

 rior, the limestone is very dark in colour, extremely hard, and makes 

 a remarkably useful lime for agricultural and other purposes ; while 

 the limestone from Poolvash, in the centre of the basin, is very 

 light coloured, rather soft, and is not equally convertible into lime. 

 In other parts it is altered by the intrusion of Trap rock into a pure 

 dolomite, as at Scarlet and Strandhall. In the N.E. corner of Cas- 

 tletown Bay it is of a brown arenaceous character, and highly crystal- 

 lized in texture. A comparison of the fossil contents of tnese beds 

 also indicates great dilferences in the physical conditions under which 

 they were deposited. In the dark-coloured Innestone of Derby- 

 haven, etc., organic remains are comparatively rare, while in the 

 light-coloured they are, in most places, so numerous as to form fully 

 two-thirds of the substance of the rock itself, and to give it its cha- 

 racteristic light colour. Out of a list of 222 species of Manx Car- 

 bom Fim-ous fossils published by Mr. Cumming, only 76 species are 

 found in the lower or dark limestone, while 153 are found in the 

 upper or light-coloured beds. Eurther, these two series of rocks 

 " have comparatively few species in common, and those which are 

 comnum are mostly such as have a great vertical range." Out of 

 the 222 species collected by Mr. Cumming, only 30 or 7'4 of the 

 whole are common to the two series of rocks. Hence Mr. Cumming, 

 when investigating these beds in 1848, was led to divide the lower 

 DUMuber of the Manx Carboniferous rocks into two groups — the upper 



