Vol. 51.] THE COUNTRY AROUND FISHGUARD. 183 



which the cavities also have been infilled with carbonate of lime, 

 near Fishguard Harbour, greatly resemble at first sight these rocks 

 in the field. The intrusive rocks are, however, readily recognized 

 under the microscope [78], [49], [65], [15], and [88]. 



Secondary quartz is found in several localities in considerable 

 abundance in the intrusive rocks. It occurs as irregular grains or 

 patches, replacing either portions of the groundmass or the original 

 crystalline constituents [71], [60], and [350]. On the northern coast 

 of Strumble Head at Aber Morgan, the diabase is of light-grey 

 colour and has a subconchoidal fracture mainly due to this infil- 

 tration of quartz [275] ; and in a tough, violet-grey, fine-grained 

 rock from tbe cliff's near Globo Pawr [251] the silica not only re- 

 places much of the groundmass and some of the felspar-crystals, but 

 forms spherulites and fills cracks and vesicles to such an extent as to 

 render the rock totally unrecognizable in the field as a diabase. The 

 quartz-filled vesicles weather out on the surface like white pebbles. 



Leucoxene is a common alteration-product of the ilmenite, and 

 pyrites is to be seen in greater or less quantity in nearly every 

 specimen of diabase. 



(3) The Variolitic and other Varieties of the Intrusive Mocks. 



There are several exposures of two closely- allied types of intru- 

 sive rock which have many points of resemblance with tachylytes 

 and variolites, and merit careful description. 



The glassy or tachylytic variety possesses a peculiar ' slaggy ' 

 appearance under the microscope. Some of the best examples come 

 from Garn Fawr near Pwll Deri. In the field and to the unaided 

 eye the rock merely resembles a very fine-grained compact ' green- 

 stone ' ; but under the microscope we see, in slides [53] and [101], 

 pale green granules arranged in lines in single or multiple series, 

 and more or less fused with each other into rod-like structures ; 

 these rods lie in groups parallel one to the otber. The relation of 

 the length of an individual rod to its breadth is on an average 

 about 30 : 1, while the separate rods frequently show curvature, and 

 are furnished with short parallel branches which arise at an acute 

 or right angle to the main stem. The groups of parallel rods lie at 

 all angles to each other, and each group consists of two to eight 

 rods. Detached granules with or without a linear arrangement 

 represent the rods in other parts of the groundmass ; and all stages 

 of the fusion of these separate granules — ' globulites ' — into one 

 of the rods or branches may be seen. Some of the parallel rods 

 thus composed are stumpy, and the proportion of the length to the 

 breadth may be as low as 3 : 1. Minute pale green granules of the 

 same nature are scattered profusely throughout the groundmass. 

 These ' globulites ' and the long rod-like ' crystallites ' which they 

 build up remind one of the augite-skeletons in the Arran pitchstone, 1 

 of the rod-like structures of little grains, etc., of augite in the 



1 Allport, Geol. Mag. 1872, p. 536 ; Roseiibusch, ' Mikroskop. Physiogr.' 2nd 

 ed. vol. i. (1885) pi. iii. fig. 3. 



