174 MB. E. K. COWPEK EEED ON THE GEOLOGY OE [May 1 895, 



secondary origin of amygdaloids. Simple infilled original vesicles 

 are common, and are usually represented as quartz-amygdaloids. 

 As an example may be taken a rock [231] from the volcanic series 

 on Strumble Head, north of Garn Gilfach. In this there are 

 numerous oval or lenticular patches of clear quartz-grains with 

 some included epidote. By their regular shape and the parallelism 

 of their longer axes they must be considered as original vesicles drawn 

 out in the direction of flow, and subsequently infilled with quartz. 

 Minute granules of a pale greenish colour abound in the rock. 



Beautiful instances of this well-known type are found also on 

 Cam Madog and Cam Ffoi. Sometimes the outer lining of the 

 vesicles consists of radially arranged quartz-fibres, while the interior 

 is filled with clear quartz-grains [305]. But these are not of the 

 nature of hollow spherulites (lithophysae). Incipient spherulites of 

 true secondary origin occur in the groundmass of the same rock. 

 The centre of a vesicle may be occupied by epidote, while the rest 

 of it is filled with quartz [245]. 



An interesting rock from the vicinity of Caerlem, on Strumble 

 Head, shows oval white patches dotted about in a pale yellowish- 

 green matrix [232]. Under the microscope the spherulitic type of 

 devitrification, described by Rutley in the Long Sleddale rock, 1 is 

 found accompanying the micropoikilitic structure. The latter is 

 only partially developed, patches of the cryptocrystalline ground- 

 mass and microlitic felt being still present. The spherulites, each 

 giving a distinct black cross, occur in ovoid or irregular patches 

 and constitute the oval opaque white spots. These spots seem to 

 represent original true vesicles, each of which is the centre of the 

 special type of devitrification which has obscured their outlines. 



In one of the micropoikilitic felsites [6] from the neighbourhood 

 of Ciliau, east of Llanwnda, the vesicles are lined with a very pale 

 green, transparent, granular, isotropic mineral ; the centre of each is 

 occupied by one or more crystalline patches of quartz, aud one or 

 more well-formed and perfectly terminated twinned felspar-crystals 

 are seen in the interior of several vesicles. These felspars must be 

 of secondary origin, but are earlier than the quartz-infilling. In 

 this case I do not believe that the vesicles, with their irregular out- 

 lines, can be original gas-filled cavities. It seems more probable 

 that these have been either decomposition-cavities subsequently filled 

 up by quartz, or that they are segregatory patches due to a dif- 

 ferentiation of the rock, before its final consolidation or at the 

 time of its micropoikilitization. 



' Contraction-spheroids ' are represented in a rock from Caerlem 

 [238], in which the groundmass has suffered partial micropoiki- 

 litization. In ordinary light one notices under the microscope oval 

 or irregularly-shaped areas, distinguished by their pale brownish 

 colour and abundant specks of an opaque or dark greenish dust. 

 A clearer irregular central patch is visible in all ; and the whole 



1 ' Felsitic Lavas of England and Wales,' p. 12 & pi. ii. fig. 1. 



