Vol. 51.] THE COUNTRY ABOUND FISHGUARD. 167 



coarse and freer from ' dusty ' material ; or the patches may be 

 indistinctly blended with each other, so as to give a blurred appear- 

 ance with crossed nicols. In some cases, the inclusions in the 

 patches consist entirely of microlites ; in others, principally of 

 felsitic, cryptocrystalline, or ' dusty ' material ; and in yet other 

 cases, of a mixture of these two kinds. 



It has been stated that the simple typical cryptocrystalline and 

 microlitic groundmass may co-exist even in the same microscope- 

 slide with the micropoikilitic structure ; and perlitic structure may 

 also occasionally be found with it, though in the coarser mosaics the 

 perlitic cracks tend to become effaced or very indistinct. 



It has been seen that it is the opinion of the majority of observers 

 that the structure is secondary, and a form of devitrification. From 

 the following considerations I am inclined to think that it is deve- 

 loped subsequent to the ordinary devitrification of an acid lava with 

 a glassy base ; or at any rate later than the production of the felsitic 

 matter which one is accustomed to regard as the normal result of 

 devitrification. 



In the first place, we have seen that the cryptocrystalline and 

 microlitic types of* groundmass may exist side by side in the same 

 rock, pointing to the original occurrence of patches of glassy material 

 in the midst of a rock in which crystallization was just commencing. 

 Holding the formation of microlites to be the first stage in the 

 building up of crystals, we may look upon these patches of inter- 

 lacing microlites in the midst of a glassy base as so many centres 

 of incipient crystallization. At this stage the rock consolidated ; 

 and then the glassy patches suffered the well-known change by which 

 they became felsitic or cryptocrystalline, or, in common parlance, 

 they were ' devitrified ' ; and one gets a rock such as that from 

 Cam Gelli [203] previously described. 



In the next place we find rocks either with a simple microlitic or 

 a simple cryptocrystalline groundmass, or with a microlitic-crypto- 

 crystalline groundmass, with only small patches of the micropoiki- 

 litic structure scattered about in it. These sporadic patches, each 

 behaving as a single crystalline individual, and generally with 

 a blurred indefinite boundary, contain the microlites, etc., of the 

 rest of the rock, while between the patches the groundmass exists 

 unaltered. Examples of this condition in felsites with microlitic 

 groundmass are found in slides [202], [209], [231], [70], and [318], 

 from such widely separated localities and horizons as Cam Fran, 

 Carn Gelli, and Strumble Head. Well-marked perlitic and flow- 

 structures are seen in those in which the ' micropoikilitization ' is 

 only slight [5]. Among cryptocrystalline felsites this first stage is 

 exhibited in some rocks from the neighbourhood of Goodwick [69], 

 [34] ; and in a rock from Carn Gelli [96] there is very good perlitic 

 structure preserved in an almost simple cryptocrystalline base. 



The next stage shows the micropoikilitic structure more generally 

 and regularly distributed throughout the rock. An example of this 

 stage is a banded cryptocrystalline felsite from near Goodwick 



