GENERAL REMARKS ON GNEISS. 225 



attached breccias so frequent in limestone. But this is 

 not the place, under the present dearth of information 

 on this and similar subjects, to enter into any further 

 speculations, and I shall therefore proceed to such an 

 arrangement of the individuals of this family as my present 

 state of information admits. 



BEFORE enumerating the several rocks here united 

 under the general term of gneiss, it is however proper 

 to say that the limits of the definition have been con- 

 siderably extended. This seems unavoidable, since the 

 varying composition of the rock and the gradual nature 

 of the changes it undergoes, render it impossible to apply 

 separate terms to each variety, without accumulating a 

 load of titles which would encumber instead of elucidating 

 the subject. Nor is there any reason why the definition 

 should not be so extended, since no rigidity has been 

 held necessary in the case of other compound rocks where 

 variations even greater are of common occurrence. In 

 enumerating the varieties I have however separated the 

 most anomalous from those which, with greater simpli- 

 city and regularity of structure, exhibit in a great mea- 

 sure, if not entirely, a composition corresponding to the 

 ordinary definition of this substance. It is for geologists 

 to receive or reject this arrangement as they shall see 

 right ; it can only be offered as a very imperfect attempt 

 towards the attainment of the object, since it is founded 

 on observations limited not only to Scotland but to a 

 small part of that country. 



In the first division are placed all those of which the 

 composition may be considered regular, as being identical 

 with that of granite : the second contains those that are 

 characterized chiefly by the presence of compact felspar ; 

 and the third includes all which, with a community of 

 structure and geological position, present a different variety 

 or combination of ingredients : of these last the greater 



VOL. I. Q 



