246 J.J. H. TEALL—-PETROLOGICAL NOTES ON 
The Cleveland, Cockfield, and Armathwaite dyke in composition, 
structure, and geological age is a pyroxene-andesite; and as the 
pyroxene is monoclinic, it may fairly be called an augite-andesite. 
The classification of the rocks referred to in this paper would be 
greatly facilitated if we had a ready method of determining the 
felspars. Unfortunately we appear to have no such method. I 
have tried both alterability by acid and the method of extinction, 
as recommended by Messrs. Lévy and Fouqué. The results, how- 
ever, have not been such as to give me confidence in their general 
applicability. When the crystals are sufficiently large to enable 
one to isolate fragments, then specific-gravity determinations and 
optical tests yield results which are doubtless thoroughly reliable. 
In conclusion I would remark that any system of classification, to be 
natural, should take into consideration all the characters of igneous 
rocks—chemical composition, mineralogical composition, micro- 
scopic structure, and mode of occurrence; and that in estimating 
the relative value of these characters great importance should be 
attached to the fact that the chemical composition of unaltered spe- 
cimens is that of the original magma, by the consolidation of which 
the rock in question has been produced. 
EXPLANATION OF PLATES XII, & XIII. 
Prats XII. 
Fig. 1. Cleveland dyke, centre: X50. Portion of a porphyritic felspar in top 
right-hand corner. The remainder of the figure illustrates the general 
structure of the ground-mass. Crystals of felspar, grains and 
granular aggregates of augite, magnetite, and interstitial matter may be 
recognized. 
2. Cleveland dyke, margin: X50. Broken and corroded felspar in the 
top right-hand corner. General structure of the ground-mass shown 
in the remainder of the figure. Compare with fig 1. 
2a. Armathwaite dyke: 350. Portion of glassy base showing a small 
augite surrounded by longulites and.globulites. Longulites formed 
_ by the coalescence of globulites. The. small augite runs out into 
microlites. 
3. Cleveland dyke, Presten: x20. Large felspar zoned by inclusions 
and penetrated by inlets of the ground-mass. 
4a. Cleveland dyke, Preston: 170. Skeleton felspars in the ground- 
mass. 
4, Cleveland dyke, Preston: x170. Structures observed in the interstitial 
matter. 
5. Hett dyke, Hett village: x40. [Illustrating structure of the Hett 
and related dykes. Augite, felspar, and magnetite make up almost 
all the whole mass of the rock, and may be recognized in the figure. 
A patch of augite granules occurs in the centre. 
6. Brunton dyke, Bingfield: x40. Illustrating structure of the ground- 
mass of the Tynemouth and-related dykes. The figure shows long 
narrow felspar sections, irregular grains and granular aggregrates of 
augite, and interstitial matter, rendered nearly opaque by imper- 
fectly individualized magnetite. 
