Vol. 51.] THE COUNTRY AROUND FISHGUARD. 165 



of groundmass that it is difficult to find even one slide in which the 

 two do not co-exist. This third type is the micropoikilitic. The 

 name is applied to that ' blurred-mosaic ' groundmass which is the 

 commonest condition in the lavas of this area. For many reasons, 

 which will be adduced later, it appears not to be an original structure, 

 but of secondary origin, subsequent to the consolidation of the rock, 

 and even in some cases later than the development of the crypto- 

 crystalline or felsitic condition. It is of special interest and signifi- 

 cance to find here traces of a change having taken place after the 

 rocks had suffered devitrification. However, I am inclined to ascribe 

 it to contact-metamorphism produced by the numerous sheets of 

 intrusive material and laccolites in the neighbourhood. 1 



The name ' micropoikilitic,' in the first place, needs some explana- 

 tion. The author of the name ' pcecilitic,' as applied to a minute 

 rock-structure, is Prof. E. Ha worth, who in 1888 first used it for 

 a new type of structure of the groundmass in the porphyries of 

 Missouri. 2 He refers in this paper to an analogous structure in 

 some coarse-grained peridotites described by Prof. G. H. Williams, 3 

 in which large hornblende-crystals contain grains of olivine and 

 hypersthene. The latter author considers it to be a distinct type 

 of the holocrystalline groundmass. 



Haworth, in describing the quartz-porphyries of Missouri, says : — 

 ' In many of the fine-grained rocks the individual grains cannot 

 be seen unless highly magnified, but if the section be revolved 

 on the stage of the microscope while the nicols are crossed the 

 whole field is broken up into different areas, each of which will 

 change alternately from dark to light. A great many little specks 

 of felspathic material are scattered through the whole mass, but 



they seem to be entirely irregular in their arrangement 



As the crystallization increases these different areas increase in 

 definiteness, so that in the more coarse-grained varieties their true 

 character can readily be determined.' Some of the areas have 

 central quartz-crystals with a large surrounding portion of quartz 

 in similar optical orientation, but irregularly spotted over with 

 felspathic material. Other areas have no central crystal, but consist 

 of a quartz groundmass with the felspathic material irregularly 

 distributed over the whole of it. ' In some cases the felspar is well 

 crystallized, and projects into the quartz-crystal.' The definiteness 

 and individualization of each area depend on the degree of crystal- 

 lization. Prof. Haworth considers this ' pcecilitic ' structure to be 

 original, and he has traced a whole series of transitional stages from 

 rocks with a microgranitic to rocks with a vitrophyric structure, 



1 This view of the origin of this structure has heen recently adopted by 

 Mr. Harker (Quart. Journ. Geol. Soc. vol. 1. 1894, p. 333) in the case of the 

 fragments of Eycott lavas included in the Carrock Fell gabbro. I had inde- 

 pendently formed the same view several months before Mr. Harker's paper 

 appeared. 



'■ ' On the Archaean Geology of Missouri,' pt. ii. ' American Geologist,' 

 vol. i. (1888) p. 367 and pi. i. figs. 1, 3. 



3 ' The Peridotites of the Cortlandt Series,' Amer. Journ. Sci. ser. 3, vol. xxxi. 

 (1880) p. 30, and ' The Norites of the Cortlandt Series,' vol. xxxiii. (1887) p. 139. 



