396 ARRAN. GEOLOGY. OVERLYING ROCKS. 



an intermixture of hornblende, a mineral which does not 

 enter into the composition of this substance. This is the 

 particular rock to which it would_ be convenient to limit 

 the name of clinkstone, which thus becomes the third 

 in order from claystone, compact felspar being the last. 

 The term is indeed in use for this purpose among mine- 

 ralogists, but has been also often applied to the softer 

 substance just described, as well as to compact felspar. 

 It is in truth difficult to limit names very accurately, when 

 the limits of the approximate rocks are themselves 

 evanescent. The several varieties of the clinkstone, as it 

 occurs in Arran, are sometimes massive, sometimes pris- 

 matic or even regularly columnar, while in other cases 

 they are schistose ; nor is it uncommon to find masses 

 which are columnar in one part, becoming schistose in 

 another, and even the columns themselves splitting natu- 

 rally into laminae at right angles to their axes.* 



* This variety is the porphyry slate of some authors, although that 

 uame has indeed been extended to some of the rocks of this family 

 which present no appearance of a schistose structure. As a specific 

 name, it is unfortunately chosen ; since it is founded on a circumstance 

 merely accidental, and occasionally occurring in almost all the leading 

 members of the overlying rocks. In this particular case, we have already 

 the specific term of claystone porphyry; while the schistose structure 

 is moreover but occasional, and generally occupies but a small part of 

 the whole mass; it can only be considered as an incidental variety. As 

 the same rock occasionally presents the prismatic or columnar dispo- 

 sition, it might with equal justice claim a specific term expressive of this 

 as well as of the former structure; a practice, which in the case of basalt, 

 has long been rejected by mineralogists; the form being considered 

 among the accidents of the mineral. Should the schistose structure be 

 considered a specific distinction, the same rule must be extended to the 

 other members of this family; and thus the soft blue claystone and the 

 syenite of Mull, the overlying trap of Sleat, and the veins of Isla, would 

 aspire to the rank of species. Nay, it might even be extended to granite, 

 and thus the schistose granite of Catcol must also be erected into a new 

 species by the name of granite slate. It is unnecessary to produce 

 further arguments for the propriety of rescinding the term of porphyry 

 slate from the mineralogical nomenclature. 



