:300 PROF. T. G. BON^EY AND MISS C. A. EAISm 0^ [May 1 899, 



(see p. 278) forms a narrow dyke-like mass (about 7 inches wide) 

 with rather flaky structure ; and under the microscope a specimen 

 shows alternating streaks, marked respectively by the presence and 

 absence of scattered iron-oxide.^ 



This additional evidence supports the view that the chlorite- 

 schist is probably an intrusive rock ; but we are no nearer to the 

 •discovery of its original nature." 



YII. Effects of Peesstjee; A&e of the Rocks. 



The results of the pressure which has acted on the schists of 

 Anglesey are very familiar ; but the igneous rocks which we are 

 describing show similar effects. The serpentine generally is more or 

 less schistose, as already shown by one of us,^ some specimens even 

 resembling those which have been described from the Swiss Alps.^ 

 The crushed laminae are thin, even papery, sometimes crumpled 

 but generally curving, and they then bend around more compact parts. 

 These form hard egg-like nodules which slip out easily from the 

 coating of the schistose laminae. They may vary from 2 to 3 inches 

 in length, but both smaller and very much larger examples occur. 

 One has been described near the lakes about 3 feet long (p. 277) ; 

 but masses much larger than this, bounded by curving schistose 

 laminae, or even large patches of serpentine, have in like manner 

 escaped crushing. 



The gabbro, as has been said, occurs both unmodified and 

 pressure-modified. The latter parts sometimes consist of green 

 and white, or more commonly green and yellow (because epidotic) 

 elongated spots with a distinct orientation, in which condition the 

 rock can be identified under the microscope. At other places it is 

 difficult to decide whether the mass belongs to the green schists, or 

 is a pressure-modified compact variety of gabbro. We cannot, 

 therefore, be sure that in this respect our mapping is always correct. 

 Still, the presence of crystalline patches or other evidence often 

 :gives some indication. 



The schistose structure in the gabbro is traceable over considerable 

 ureas, but is absent from others. Thus the large masses at Graig- 

 fawr and Cerig-moelion have mainly escaped, while past Tyddyn-y- 

 ■cob the rock is really a gabbro-schist. This difference may be 

 partly due to the crushing along certain thrust-planes, but it is 

 possible that, if the Graig-fawr and Cerig-moelion bosses represent 

 ;the solid cores of massive intrusions, these may have resisted where 



^ In certain specimens from here and from near Yr-ynys, chlorite-Teins are 

 associated sometimes in a brecciated fashion with patches composed mainly of 

 .epidote and (?) zoisite intimately intercrj'stallized. The rock might be akin 

 to the gabbro, but the pyroxene would be absent. Some of the structure is 

 like that of mineral veins ; here larger crystals of epidote have formed — the 

 prisms projecting from the edge of the epidote-zoisite patches into or across 

 the chlorite-band. 



^ See Geol. Mag. 1890, pp. 539, 540, for a discussion of this difficulty. 



3 T. a. Bonney, Quart. Journ. Geol. Soc. vol. xxxvii (1881) pp. 41, 43. 



^ Geol. Mag. 1890, p. 536 ; ibid. 1897, p. 114. 



