ISLE OF MAN. GEOLOGY. SCHIST. 539 



ing two circumstances which ought, as far as possible, 

 always to be distinguished in description, namely, the 

 geological connexions and the mineralogical characters, 

 or by deducing the former as a necessary consequence 

 from the latter. With respect to the particular class 

 of rocks under review, namely, the argillaceous schists 

 of the primary class, a practice has prevailed, with what 

 propriety appears doubtful, of making geological divisions 

 where Nature appears only to have made mineralogical 

 distinctions, and of founding essential characters on cir- 

 cumstances which ought perhaps only to be considered as 

 accidental. In the case of micaceous schist, a class of 

 rocks of as great frequency and extent as the argillaceous 

 schists, and exhibiting varieties in composition much 

 greater than those which occur among the latter, 

 no such division has been thought necessary ; every 

 variety occurring in this series being admitted to stand in 

 a common relation to the surrounding rocks, although 

 differing from each other in a much greater degree than 

 fine clay slate does from those coarse varieties which 

 have been designated by the term graywacke. If it is 

 judged expedient by geologists that the same practice 

 should be extended to the rocks immediately under review, 

 the confusion which has hitherto so often attended them 

 will probably be removed, and the history of the argil- 

 laceous schists reduced to that simplicity so much to be 

 desired in these investigations-. 



It is unnecessary here to repeat the different observa- 

 tions scattered throughout this work, which form the 

 arguments for the view now taken of the common relation 

 in which all the argillaceous schists stand to each other 

 and to the surrounding rocks. These are supported by 

 the examples of Cornwall, Wales, and Cumberland ; the 

 structure of which countries resembles that of the Isle of 

 Man, and in all of which the several argillaceous schists 

 form part of a common deposit. It is certain that gray- 

 wacke is often predominant, that it is even sometimes 

 exclusive ; yet, in those cases also, it maintains the same 



