TATE: YOUKSHIflE PETROLOGY. 
377 
easy to identify by its two cleavage planes and its chromatic effects, 
fills in all the vacated cavities in such abundance as to cause the 
rock to effervesce very freely. (Fig. 2S). 
To sum up, the rock then is holocrystalline, its essential con- 
stituents being orthoclase, hornblende and biotite : it is therefore a 
Mica-syenite. So far as is known it stands unique among British 
rocks. Its nearest allies lie in the mountains of the Vosges. We may 
compare, for example, the specimen and slide, No. 582, in the 
Museum of the Geological Laboratory, Royal School of Mines, 
S. Kensington : described as a Mica-syenite from Cleury, Vosges. 
Omitting paragenetic indications, this specimen may be regarded as 
typical of what our rock may have been in its pristine purity ; and 
if, as is stated, the Vosges Mica-traps perceptibly alter the rocks 
with which they come in contact, the analogy may be further 
extended inasmuch as our Mica-syenite dyke metamorphoses, the 
calcareous shales towards which it hades, into a highly-crystalline 
limestone band, some two inches in thickness. 
(2) On an inspection of the six -inch Ordnance Survey Sheet, No. 
96, it will be seen that,'about 350 yards below Pecca Steps, Thornton 
Beck bends eastward towards Manor House. The bed of the stream 
is formed in part by a trap dyke, extending for several yards from 
the east end of this bend. It is difficult of access save when the 
water is very low, only a small portion of the dyke lying exposed, 
under a tree, upon the north bank of the stream. 
Macroscopically, this rock has a somewhat fullers-earth like 
appearance, due to the mixture of a greenish-grey, with a creamy- 
coloured earthy decomposition product, the former probably after 
biotite, the latter felspar. Along the southern boundary of the 
dyke, it is of a steel-grey tint, the plates of black mica here being 
still visible. The calcareous shales in contact therewith have assumed 
a crystalline aspect. 
Microscopically, along with a profuse development of biotite in 
a matrix of grey decomposition products, we have in abundance 
sections which, at first sight, suggest pseudomorphs after Olivine 
(Fig. 16, 17) and Nepheline (Fig. 18.9. See also Fig. 2). There are 
no indications of hornblende by reflected light, nor can any outlines 
of felspar be made out. We shall return to this shortly. 
