162 ME. E. R. C0WPEK, REED ON THE GEOLOGY OE [May l895 ? 



There is firstly the cryptocrystalline type. This is comparatively 

 rare in a perfect and unassociated state, and it is not easy in all 

 cases to determine amongst rocks of this class whether we are deal- 

 ing with lava-flows or minutely comminuted silicified ashes, since 

 both have the same hornstone-like appearance. The best examples 

 of lavas with this type of groundmass occur in the neighbourhood 

 of Goodwick. A rock of a very dark bluish-grey colour, which 

 comes from the shore below that village, exhibits under the micro- 

 scope numerous lath-shaped and squarish sections of felspar-crystals, 

 granophyric aggregates of quartz and felspar, some probably of a 

 porphyritic nature, others undoubtedly subsequent to the intra- 

 telluric period when the porphyritic felspars and quartz-crystals 

 were formed, for some of the clear rounded quartz-crystals are 

 surrounded by a secondary granophyric growth. Other small, 

 rounded, clear grains of quartz are without this secondary ring, 

 while others have corroded outlines ; and there are the interesting 

 micropegmatitic groups which are described immediately below. 

 These features are exhibited in slides [76] and [21]. 1 



The felspars usually belong to the potash series and monoclinic 

 system, but in a rock from the road above Goodwick [67] a large 

 number of the porphyritic elements show albite-twinning. 



The micropegmatitic structures are especially striking in some of 

 these rocks [21], [42], [76] (PL VI. figs. 3, 4, 5). Their peculiar 

 appearance at once catches the eye. Prom a central or subcentral 

 point in a clear, crystalline, definitely circumscribed patch, blunt, 

 reticulating, and branched processes radiate outwards in all directions 

 towards the bounding-faces of the crystal or borders of the crystal- 

 group. Sometimes these clear patches have the outlines of felspar 

 and at other times of quartz-crystals ; but usually they are more or 

 less irregular in shape. They are clearly marked off from the crypto- 

 crystalline groundmass. Under crossed nicols they are seen to 

 consist either of a single individual crystal containing the radiating 

 processes, or more commonly of a group of a few crystalline grains, 

 differently orientated, irregularly aggregated, and penetrated indif- 

 ferently by the same processes. 2 In fact, Prof. Judd's term 

 ' glomero-porphyritic ' might be suitably applied to these groups ; 

 and the appellation ' glomero-porphyritic micropegmatite ' has been 

 suggested to me by Mr. H. Kynaston, P.G.S., who has found similar 

 structures in some Cheviot quartz-porphyries. They are undoubtedly 

 original structures, and belong to the same period of formation as 

 the porphyritic felspars. A granophyric group is described by 

 Iddings 3 in a rhyolite from Eureka, Nevada, which is very similar to 

 the Goodwick structures : and Mr. If. Woods, P.G.S., has shown 

 me a slide from a Builth rock of Ordovician age with the same 



1 In this paper the numbers in square brackets are those of the niicroscope- 

 sLides in the Woodwardian Museum. 



2 In the lava-fragments contained in one of the Goodwick breccias [27] these 

 structures are of unusual beauty. 



3 Seventh Annual Eeport (1886), U.S. Geol. Surv., ' Obsidian Cliff, Yellow- 

 stone Park,' p. 275. 



