208 E. HILL AND T. G. BONNEY ON TEE 



larger crystals. Felspar crystals are numerous, but so much altered 

 by the formation of secondary minerals that it is not safe to con- 

 jecture their origin. 



Breccia, Bardon Hill, Upper Quarry (p. 781). — The slide shows 

 a good deal of epidote, both in microliths and larger crystals, and a 

 quantity of an earthy-looking granular dust ; but on crossing Nicols 

 an imperfectly crystalline ground-mass is seen, the bright patches 

 being of irregular outline and form. Still the structure can be 

 paralleled in some of the Hungarian trachytes ; so that this rock also 

 may be of igneous origin. There are occasional larger felspar 

 crystals, but these are more or less replaced by epidote. 



Breccia near Summit ofBardon Hill (p. 781). — This rock is rather 

 similar to the last, except that it is more porphyritic, and the dusky 

 granules are irregularly gathered together, so as to give it a frag- 

 mentary aspect. This, however, to a great extent, disappears on 

 crossing prisms, and this rock too may be of igneous origin. It is 

 doubtless difficult (owing to the extent of secondary metamorphism) to 

 be quite certain of the origin of these fragments, but they are most 

 probably devitrified quartzless orthoclase-oligoclase trachytes. The 

 correspondence of the High-Towers specimen with those from Bar- 

 don, strengthens the reasons for referring the rocks of the latter to 

 some part of the former series. A rather similar fragment from the 

 agglomerate of Ratchet Hill is distinctly quartziferous, and contains, 

 according to one analysis, Si 2 = 78*9 ; according to another, 

 Si0 2 = 79*l. These were kindly made by Mr. Houghton, Scholar of 

 St. John's College. Its difference from the associated darker frag- 

 ments (described above) is accordingly not very great, being chiefly in 

 crystalline condition. 



Breccia of Whitwick (p. 779). — Fragments of a pinkish to purplish 

 rock of rather mottled aspect are common in this breccia ; three of 

 these have been examined. In two of them oval or lenticular 

 patches, whose texture sometimes differs a little from the rest, are 

 outlined by rather closer rings of darker granules, the ground-mass 

 being composed of very minute microliths irregular in form and dis- 

 position, and brown or black dust, in an apparently glassy base. 



These rings with crossed Mcols are seen to be associated with the 

 bright hornblendic mineral already mentioned. On applying a 

 selenite plate the above distinction is often made more clear, the 

 colours within and without the mass being complementary. The 

 longer axes of these spots are roughly parallel, indicating probably 

 an imperfectly cleaved structure of the rock. The third fragment 

 seems to show a similar structure; but has been more highly 

 affected by secondary metamorphism, epidote, viridite, &c. being 

 largely present. These rocks, on the whole, recall to mind the 

 texture of the slates already described rather than that of true 

 igneous rocks, but it is difficult to form any certain conclusion. 



It now remains to discuss the origin of the larger felspar crystals 

 present in many of the above rocks, and of the less frequent quartz- 

 grains. The evidence on both these points is conflicting, and it is 

 very difficult to arrive at a conclusion. The various slides have 



