GRIN'DLET — GEOLOGY OF THE ISLE OF MA.^. 



177 



or liixlit-coloured limestone of Poolvasli, and the lower or dark- 

 coloured limestone of Derbybaven, etc. Tbe list of the Manx Car- 

 boniferous fossils upon which this subdivision of the Manx limestone 

 into two groups is principally based, is of course very far from being 

 complete ; we have in our own collection several not included in it, 

 and undoubtedly a more careful search would disclose many more ; 

 while several designated in it as species have since been reduced to 

 the condition of mere varieties. Yet, on the whole, it is a trust- 

 worthy record, and the minute observations accompanying it are 

 carefully made and generally accurate ; we feel, therefore, no diffi- 

 culty in accepting this proposed division of the lower portion of the 

 Manx Carboniferous limestone into at least two distinct groups. 



Subsequent to the deposition of tbe Poolvash beds a great change 

 seems to have taken place in the physical condition of this part of the 

 southern basin. The strata were violently distiu'bed along a line of 

 fault parallel, or nearly so, with the present coast-line, and a consi- 

 derable outburst of trap took place, which has greatly altered the 

 underlyirg limestone, in some places converting it into pure dolomite. 

 The extreme violence of this outburst, however, seems to have soon 

 abated, after which the volcanic ash was poured out so quietly as 

 not to interfere to any great extent with the ordinary operations of 

 organic life — organic remains being found imbedded in it as regu- 

 larly as in the limestone. At the same time that this submarine 

 eruption was thus forming in one part of the basiii a thick bed of 

 volcanic ash, a deposit of black carbonaceous mud was also being 

 formed in another part, and these two dissimilar sources by ming- 

 ling have formed a very curious and interesting rock, one or other 

 of the two ingredients predominating according to the varying cir- 

 cumstances of the case. " At one period the carbonaceous deposit 

 seems to have entirely prevailed ; perhaps the volcanic action entirely 

 ceased, gathering strength for a subsequent eruption. The bed then 

 formed has its own lithological character and fossils ; it is the black 

 Poolvash marble." After a time, however, this period of quiet de- 

 posit was abruptly terminated, and the volcanic action renewed with 

 increased violence; thetrappean and mixed beds already formed were 

 violently broken up, reduced to a fragmentary condition, and mixed 

 up with a fresh outpouring of volcanic matter ; and a sort of " trap 

 breccia" was formed, in which the imbedded fragments of the older 

 rocks are considerably altered by heat. 



Such is a brief and very imperfect account, agreeing mainly with 

 Mr. Cumming's, of a remarkably interesting series of rocks over- 

 lying the Manx Carboniferous limestone. Commencing near the 

 Slack of Scarlet, a huge mass of columnar basalt, insulated at high 

 water and forming the S.IY. horn of Castletown Bay, stretching 

 along the south coast to Poolvash Bay, a distance of about two miles, 

 they include, in addition to the imbedded limestone of pure or mixed 

 origin, almost every variety of igneous trap, from the light porous 

 ash to the hard columnar basalt, and present an appearance wild and 

 rugged beyond description. 



VOL. V. 2 a 



