SOME ANCIENT AND MODERN VOLCANIC ROCKS. 409 



suit is as follows. Some have a decidedly crystalline structure, with 

 oftentimes a distinct flow of the small crystalline needles around 

 the larger crystals, and contain both triclinic and monoclinic felspar, 

 magnetite, and augite in greater or less abundance, with perhaps 

 some pseudomorphs after olivine. Others have the acicular struc- 

 ture almost but seldom quite lost, and exhibit for the most part a 

 felsitic base, in which the same minerals as before are scattered in 

 varying quantity, together, very frequently, with garnets. These 

 more felsitic-looking traps occasionally contain a large quantity of 

 augite, sometimes in good twins. 



At first I was inclined to think that the more felsitic examples 

 might be due to an alteration of originally more crystalline beds ; but 

 since they are sometimes found to alternate with the latter upon the 

 same hill-side, perhaps the more truthful supposition would be that 

 the separate flows were often of somewhat varying nature. Of the 

 analyses given above, the one (Brown Knotts) represents a highly 

 crystalline example, and the other (Iron Crag) a rock of the more 

 felsitic-looking class. 



b. Cumberland Ashes and Felstone-like rocks. 



The ejected material associated with the old lavas presents great 

 variety in the size of the component fragments, and in the degree of 

 alteration to which it has been since subjected. 



1. Typical Ash, Steel Fell. — Pig. 10 is an example from a slice 

 of well-bedded ash- rock, seen under polarized light with the Nicols 

 crossed. The dark part consists of very fine matter, and the frag- 

 ments are mostly felsitic, lying with their long axes parallel with 

 the planes of bedding. Among the fragments, crystals of felspar and 

 greeu pseudomorphs after augite are not unfrequent, some of them 

 quite as perfect as many crystals in the traps. 



2. Altered Slate, Capel Curig. — In order that correct conclusions 

 might be drawn with regard to the structure of these altered ashes, 

 I have, for comparison, examined the slate immediately beneath the 

 intrusive greenstone she'et of Craig Wen, near Capel Curig. It has 

 been altered into an exceedingly compact, almost felsitic-looking 

 rock, weathering white along the edges, like felstone. When viewed 

 with a high power it is found to consist of very minute granules, 

 with darker patches here and there, made up of the same, very 

 closely aggregated. Under polarized light, with the prisms crossed, 

 the ground is dark, with scattered points of light, caused by the 

 intermixture of doubly refracting particles. 



Now the very finely granular part of the ash represented in fig. 10, 

 has precisely the same structure as the fine altered sedimentary rock. 

 If then we find other rocks in this ashy series looking compact and 

 felstone-like, yet having the same minute granular structure as is com- 

 mon to the altered slate of Craig Wen and the recognizable fine ash, 

 we are surely entitled to conclude that the rocks in question consist 

 of exceedingly fine ashy or other matter, and are not true lava-beds. 



3. Bleaberry Fell. — The example I will first take of these doubt- 

 ful-looking rocks is from the summit of Bleaberry Fell, near Keswick, 



