176 
THE GEOLOGIST. 
jV.E. of Castletown, where the Old Eed is seen resting unconforra- 
ably upon the slaty schists as a eonglon^erate 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. 
Kesting conformably upon the Old Hed 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 liuiestone 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 Per- 
wick Bay, a little south of Port St. 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 Lauguess. 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 advantage. 
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 rema]'kably 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 tuese beds 
also indicates great difl:erences in the physical conditions under which 
they were deposited. In the dark-coloured limestone 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- 
boniferous 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. Further, these two series of rocks 
" have comparatively few species in common, and those which are 
common 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 
member of the Manx Carboniferous rocks into two groups — the upper 
