"Vol. 51.] THE COUNTRY AROUND FISHGUARD. 191 



are sharply marked off from the surrounding matrix and have the 

 lath-shaped felspar-crystals of the groundmass arranged tangentially 

 round them, occur in a diabase from Cwm Felin, north of Llanwnda, 

 [271]. These patches are in fact nodular structures. They seem 

 to be segregatory patches of the interstitial matter, and are generally 

 completely free from any of the crystalline elements of the ground- 

 mass, but the lath-shaped felspars project into the outer portion of 

 the patches when they are not sharply marked off. When the whole 

 of a spherical or oval nodule has been removed by solution, and the 

 ■cavity thus arising is filled up with cryptocrystalline silica, calcite, 

 dolomite, etc., they have exactly the appearance of infilled gas-vesicles. 

 But the co-existence in the same slide [271] of these amygdaloids in 

 all stages of formation and of nodules partially removed and replaced 

 at once shows that they cannot be pseudomorphs after gas-bubbles, 

 such as have been described by Teall 1 in the case of the Tynemouth 

 dyke. 



The spherical amygdules in the outer coats of the spheroids of the 

 diabase near Aber Clawdd Pridd seem, however, to be infilled vesicles, 

 once occupied by gas. Miss Raisin, in her previously quoted paper 

 on the Lleyn variolites, mentions vesicles occurring under similar 

 conditions. 



(4) Specific Gravity of the Intrusive Rocks. 



There are some interesting points connected with the specific 

 gravity of these rocks. The most crystalline members have, as we 

 should expect, a higher specific gravity than the tachylytic and 

 variolitic varieties. Thus the holocrystalline rock [108] of Henner 

 School has a specific gravity of 3'00 ; it is practically a gabbro. 

 Other rocks not quite so coarsely crystalline, [28], [107], and [15], 

 vary in specific gravity between 2*93 and 2'94 ; while in the 

 variolitic and tachylytic varieties [257], [226], [53], and [101] the 

 specific gravity ranges between 2*67 and 2"68. But in each case, 

 when porphyritic felspars are abundant, the specific gravity rises, as 

 for instance in the Garn Fawr rock [287], where it is 2-70. The 

 extraordinarily high specific gravity (2'93) of the tachylytic rock 

 [263] from Caerlem and the still higher specific gravity (3-04) of 

 the perlitic tachylyte [262] from the same place do not seem due 

 to the secondary addition of iron pyrites, but to some process of 

 segregation of the heavier more basic magma to the margin of the 

 intrusive mass. 2 Bocks which consist almost wholly of felspar, 

 such as [350], and might be classed with Giimbel's leucophyres, 

 have naturally a lower specific gravity, as for instance (2*66) in the 

 rock from Bhos Hes Cwm [350]. The Penrhiw nodular and vesi- 

 cular rocks [40], [31], and [224] have a specific gravity of 2'70. 

 The perlitic marginal modification [77] of these has a specific 

 gravity of 2-92 — another instance of the diffusion to the margin of 

 the more basic part of the magma. 



1 Geol. Mag. 1889, p. 481 & pi. xiv. 



- The recent paper by Mr. Harker on the Oarrock Fell gabbro bears out this 

 suggestion (Quart. Journ. Geol. Soc. vol. 1. 1894, p. 311). 



r2 



