CANNA. GEOLOGY. 459 



appeared among many other records of the former state 

 of the globe.* 



Canna has long been celebrated for the disturbance 

 which its rocks produce in the magnetic needle ; but 

 this circumstance is equally remarkable in most of the 



* Having expressed some doubts on the propriety of the use of 

 the term trap tuff as applied indiscriminately to all these breccias, 

 a few remarks on the use of this term will not be esteemed improper. 

 The original term, tufo, is applied by the Italians to loose or stony 

 aggregates of volcanic sand or volcanic fragments. From this original 

 sense it has been so far extended as to include at present the 

 following discordant substances : calcareous incrustations and deposits, 

 certain loose sandstones found connected with trap rocks, decomposed 

 basalt or greenstone, basaltic angular breccia, and basaltic conglomerate 

 bearing the marks of mechanical attrition. There is no greater enemy 

 to sound reasoning than a lax use of terms ; and it is no small meta- 

 physical evil that laxity of nomenclature is too apt to lead to laxity 

 of thinking, and to produce consequent careless habits of observation 

 and reasoning. Moral science bears evidence perhaps even more 

 striking than physical, of the evils arising from the abuse of terms 

 alone ; although the history of the latter for a period of many centuries 

 is not deficient in this respect. The science of geology owes no 

 small share of the rapid progress it has lately made, to a more 

 precise language; and it is desirable that such precision should 

 still be extended where it may appear wanting. If the term tufo 

 be considered as generic, it is evidently inapplicable to substances 

 so discordant as those above enumerated, nor is there any reason 

 why a conglomerate rock formed of granite or schist should be 

 designated by a term different from one formed of basalt, since 

 similar causes have conduced to the formation of both. The generic 

 distinction is in the one case derived from the mechanical disposition 

 and not from the mineralogical character, and it is equally and 

 properly applicable to both. The specific distinction of those con- 

 glomerates which consist of trap rocks, may be readily made by 

 specifying the component parts. If the name of trap tuff is to be 

 retained it should be limited to those accumulations of matter re- 

 sembling real volcanic tufo which are well known as occurring in 

 the trap rocks. By this method both uniformity and accuracy will 

 be introduced into the language of geology, as far as respects these 

 substances. 



