Feathers ionhaugh^s Geological Repoi't. 17 



ten state in vast quantities ; these streams of lava, when 

 indurated, become rock again, of various degrees of mine- 

 ral composition, and whose constituent parts have a great 

 affinity to the simple elements of the rocks composing the 

 mass of the inorganic rocks before referred to. In examining 

 the strata of the superior part, we often find them penetrated 

 and disturbed by rocks of this character, bearing the general 

 name of trap. It is inferred from them, and from their ap- 

 pearance, even in the inferior rocks, where also granitic 

 veins are found injected, that, in the earliest stages of the 

 crust of the earth, igneous causes were in action of sufficient 

 force to fuse mineral bodies, and to throw them towards the 

 surface, during which progress they penetrated the beds 

 which lay in their way, as is exemplified in diagram No. I, 

 of a singular exhibition of trap injected into sandstone, at 

 Trotternish, in the Isle of Skye.* It will be observed that the 

 horizontal trap veins represent the handle and triple prongs 

 of a fork, and that if all that part of the section to the left 

 from where the handle is joined to the prongs had been dis- 

 integrated and worn away in the lapse of time, leaving the 

 part to the right representing the prongs, or if the same 

 part of the section had been so covered up with other mineral 

 matter as to defy examination, the part exposed would have 

 presented a very puzzling case of horizontal trap veins ; but 

 we can here trace the prongs to the handle, and the handle 

 to a huge vertical dike of trap that has its undoubted origin 

 from below. 



Before the geological column spoken of is described, a few 

 remarks may be offered on the current nomenclatures which 

 have been applied to its various members. It was necessary 

 in the infancy of the science, to give names to the strata as 

 they were recognised ; these were, of course, either theoreti- 

 cal or local, as a natural and philosophical classification and 



* McCullccli'i Western Isknds. 



2 



