164 ME. F. E. C0WPEE EEED ON THE GEOLOGY OF [May 1 895, 



plainly an original flow-structure, and is not due to such belts of 

 minute spherulites as are the cause of similar macroscopic features 

 in some felsites. The specific gravity of this rock is 2-60. 



The second variety is conspicuous on account of the occurrence in 

 it of small nodules, which measure ^ inch or more in length, and 

 show as white oval patches on a fresh surface (see above, p. 160). 

 This variety is found only in the cliffs under Goodwick village and 

 on the slopes of the hill behind it. The nodular structure does not 

 appear to mark a special bed of lava, but to be locally developed in 

 the ordinary banded felsite of these cliffs. A few scattered nodules 

 first occur, and these rapidly increase in numbers to the maximum 

 when they constitute the bulk of the rock. There is no reason to 

 think that these nodules are of secondary origin. A study of them 

 under the microscope supports the idea that they are rather of 

 concretionary origin. But the word ' concretionary ' is so associated 

 with the thought of attractive centripetal force that it is misleading 

 here, since a centrifugal force seems to have been at work driving 

 away from definite centres the colouring-matter of the matrix in a 

 manner analogous to that in the formation of ' spots ' in slates 

 undergoing contact-metamorphism. A slight difference in composi- 

 tion — probably a higher percentage of silica — is indicated by the 

 greater resistance to weathering offered by the nodules, so that they 

 project as rough lumps on an exposed surface. Under the micro- 

 scope this rock [21] shows numerous small lath-shaped felspars, with 

 lamellar twinning, scattered through the groundmass, and irregular 

 micropegmatitic groups of the types already described. Ovoid aggre- 

 gates of clear quartz-grains mark the centre of many of the nodules, 

 which are more transparent than the other parts of the rock and 

 freer from porphyritic crystals. They are also devoid of the streaky 

 flow-structure of the groundmass. The specific gravitv of this rock 

 is 2-62. 



The second type of groundmass may be called the microlitic, 

 and represents the ordinary trachytic groundmass of rhyolites and 

 trachytes. These rocks are less acid, owing to the absence of large 

 porphyritic quartz-crystals. 



The groundmass is composed of a felt of microlites, of several 

 degrees of fineness and corresponding to Rutley's ' spiculse.' In 

 some cases they seem roughly orientated into groups, or cross at 

 right angles ; but generally they are densely interlaced without 

 any order. 



The rocks in which the best examples of this type of groundmass 

 occur commonly contain large porphyritic felspars — a distinctly 

 intratelluric generation — usually of orthoclase with corroded angles, 

 but occasionally of plagioclase with albite-twinning. Good ex- 

 amples of this microlitic type are found on Cam Fran [104], [100], 

 [201], [45], and on Cam Ffoi near Newport [312], [338]. In the 

 last two slides the felt is composed of extremely fine microlites, and 

 the triclinic felspars belong to the oligoclase-albite series. 



The microlitic type is so generally associated with the third type 



