474 EEV. J. F. BLAKE ON THE 



A third mass of granite occurs towards the east, behind the farm; 

 called Monachty. Here also the surrounding rock, away from the 

 immediate contact, is an ashy pelite, and the line of junction is clean ; 

 but in the boss of rock exposed to the east of the farm the granite 

 may be seen passing in strings into the pelite. It is sometimes 

 granophyric in structure, and the quartzes have been s^ery much 

 squeezed. There is occasionally a difficulty in drawing the line- 

 between the two rocks, owing to the granite having been brecciated 

 in situ and the cracks filled with mica, while the pelite itself 

 develops mica in the neighbourhood of the granite. 



These observations leave no doubt on my mind that the granite is 

 an intrusive rock, of the same geueral age as the ashes into which 

 it intrudes, related as the dykes of Etna might be to the mass of 

 the mountain. And the so-called schists are alteration-products of 

 the ashes, due practically to contact-metamorphism acting on 

 material of a less sorted kind than usual. Later observations will 

 be found to confirm this and yield even more decisive proofs of in- 

 trusion ; yet the rock, the origin of which is thus determined by field- 

 observations, is of the same character, presents the same differences 

 from ordinary more recent granites, and has the same felsitic asso- 

 ciates as at St. David's or Caernarvon, where the stratigraphy leaves 

 it open to geologists to account for these difierences by considering 

 it an altered sedimentary rock under the name of " granitoidite." 



There is no very determinate order of sequence in these rocks,, 

 and some of these central masses may be older than the circum- 

 ferential ; but when the volcanic debris is mixed with the chloritic 

 schists, it is with their uppermost part, and even in this corner the 

 most slaty portions, which may be considered in part sedimentary, 

 occupy the lower positions. We may therefore conclude that the 

 volcanic group forms the higher portion of the series, and the granite 

 is therefore one of the youngest rocks in the district. 



With regard to the relations of the series to the Ordovician, 

 they are not always brought together by faults. There is reason to 

 believe, as already stated, that the capping of the Gam is really a 

 beach-breccia; but more decided and ordinary Ordovician con- 

 glomerates are found on the slopes of Pen-bryn-yr-Eglwys. These 

 are met with on the road from Monachty, where it turns north to 

 go to Porth Geon. They are of various kinds. One portion, which 

 lies upon an old agglomerate, is made up of its pebbles rounded, and 

 approximates to it in appearance *. Another portion is of large 

 quartz-pebbles with ferruginous matrix, the size gradually decreasing 

 till they are the size of peas, or less ; and another is a compacted, 

 perhaps less weathered, hard, bluish rock, not at all unlike that at 

 Twt Hill. These may be traced on the ground over a limited area, 

 bounded on two sides by faults, and passing up into brown grits, 

 and then into hard blue slate. I have not found fossils in these rocks, 

 but their general resemblance to the basal Ordovician in the rest of 



* Very much as is the case with the agglomerates and conglomerates on: 

 Clegyr by Llyn Padarn. 



