Yol. 51.] THE COUNTRY ABOUND FISHGUARD. 17& 



But this rock [260] shows, under the microscope, other features 

 which exactly correspond with those in a nodular rock from Pen y 

 Chain, Pwllheli, associated with pyromerides. 1 In ordinary light 

 under the microscope it shows a finely-granular, dark olive-green 

 matrix, in which lie more or less circular areas marked out by 

 fairly broad transparent lines, and divided up into circular or 

 polygonal patches by similar lines ; these patches are of the same 

 granular character as the matrix ; and each circular area curiously 

 reminds one of a sporocarp containing spores, or of encysted 

 gregarines. There are also visible clear vesicles having their centre 

 filled with a clear greenish or violet-coloured substance, or with 

 dirty-brown matter containing a few porphyritic felspars. With 

 crossed nicols there are no outlines to the vesicles visible, but the 

 greenish or violet centres are still plain. A few porphyritic 

 felspars, twinned on the Carlsbad type and with rounded angles, 

 occur in the matrix, which is for the most part cryptocrystalline, but 

 here and there has become microcrystalline and minutely spherulitic. 

 Irregular shreds and patches of a clear olive-green substance occur 

 in it. There is a trace of phosphate of lime in this rock, suggesting 

 the presence of a little apatite. 



A mottled or ' blotchy ' rock from Coch y Ceiliog was analysed, 

 owing to the supposed occurrence of pinite in it. The rock is an 

 altered coarse tuff, and the analysis (D) gives the above result. 



The specific gravities of the lavas vary between 2*60 and 2*76, 

 but most of them are about 2-63. The micropoikilitic varieties 

 have a slightly higher (2*65) specific gravity, but no definite relation 

 between the extent of the development of the micropoikilitic struc- 

 ture and the specific gravity is traceable, although this may be 

 owing to the fact that the same lava-flow cannot be traced through 

 the several stages, by reason of the impersistence and varying com- 

 position of each bed. 



(e) The Intrusive Rocks. 

 (1) Distribution and Structure. 



The general distribution of these rocks has already been briefly 

 mentioned, and a few details may here be added. 



The common correspondence of the strike of the intrusives with 

 the strike of the bedded series suggests for the lenticular masses 

 a laccolitic origin. But the great Strumble Head mass, reaching 

 from the extreme west of that peninsula to Ciliau in the east, with 

 the southern spur through Llanwnda, does not belong to this 

 category. 



The distribution of the intrusive masses is better seen by a glance 

 at the map (PI. V.) than by any description. 



The joints that traverse these rocks are of several types. Com- 

 monly the joint-planes break up the mass into irregular blocks of 



1 Through the kindness of Miss C. A. Raisin I was enabled to examine a 

 section of this rock. 



