On the Formation of Dendrites. 415 



four strips of ground in the west and three in the east, but the full 

 succession does not occur in any one of these outcrops. The effects 

 of compression are much marked in the purple conglomerate ; the 

 matrix is dragged out along small shear-planes, and the pebbles are 

 stretched into phacoids, slip-faulted, and their extremities tailed out. 

 A deceptive appearance of unconformity is also produced at the 

 junction of grits and shales. Fossils are found in some of the 

 rocks. The Llandeilo rocks are of importance, as they reveal 

 the existence of at least four shattered synclines, usually faulted, 

 and probably all overthrust on their northern boundaries. 



No fossils have been found in the Llanbadrig rocks, and no more 

 definite assertion of their age is possible than that they are pre- 

 Llandeilo. They are newer than, and their base may be conform- 

 able with, the Green Series ; but as fragments almost certainly 

 derived from the latter series have been found in the highest zones, 

 the relation between the two series is more likely to be that of an 

 unconformity. The Llandeilo rocks contain fragments of limestone 

 and pieces of quartzite ; but in spite of this evidence in favour of 

 unconformity between the Llanbadrig and Llandeilo rocks, the 

 basal beds of the latter are found to cling closely to the highest 

 quartzite of the Llanbadrig Group. 



The quartzites are remarkable in the fact that they frequently 

 contain patches of limestone, apparently deposited with them, and 

 that they vary rapidly in thickness from about 30 to 200 feet at 

 Craig Wen, where this variation is correlated by an inverse variation 

 in the conglomerate. Although earth-movement may be partly 

 responsible for this variation, some further explanation appears 

 requisite, and this may possibly be furnished by erosion of the 

 quartzite. The overthrusting force appears to have come from 

 a direction somewhat east of north. 



The igneous rocks are dealt with in two groups ; those older and 

 those newer than the earth-movement. To the former belong 

 granite, serpentine and its associates, and basaltic dykes. In each 

 area where they occur the serpentines are associated with masses 

 of a peculiar purple limestone not known elsewhere ; they also 

 contain bands of ophicalcite, and schistose structures are common 

 in the rocks. The later dykes belong to an acid and a basic set ; 

 the latter show some evidence of minor movement, such as faulting 

 and a little shearing. The acid dykes are microgranites, grano- 

 phyres, and quartz -porphyries. In some cases the dykes are 

 composite, the acid material being the older and the basic the 

 later constituent. As a rule, the basic material invades the edge 

 of the dyke ; but in one case it has invaded the joints and cracks 

 and caught up fragments of the acid rock. 



3. ' The Formation of Dendrites.' By A. Octavius Watkins, 

 Esq., A.E.S.M., F.G.S. 



If two plane-surfaces be separated by a film of suitable plastic 

 material, and one surface be rotated slowly on the other through a 

 small arc, the plastic material collects into branching forms similar 



