232 



PROF. S. H. REYNOLDS ON A SILURIAN [May I907, 



crysts are smaller than those generally occurring in the Moon's-Hill 

 rock, and consist only of felspar, pyroxene not being met with. 

 A slice cut from a specimen taken from the top of the quarry 

 shows, however, numerous bastite-pseudomorphs after rhombic 

 pyroxene and a smaller number of felspars. Elow-structure is 

 very well shown in both sections. (See PI. XVIII, fig. 2.) 



When followed away from Sunnyhill Quarry in the direction of 

 the l Waggon & Horses ' Inn, the rock is seen to become less fine- 

 grained, and phenocrysts of pyroxene and plagioclase assume 

 prominence. 



The most abundant rock on the south-eastern slopes of Beacon 

 Hill is of a coarse grain, with relatively more abundant felspar- 

 phenocrysts than any other rock from this part of the district. The 

 very low (5°) extinction-angles of some of the felspars are sug- 

 gestive of oligoclase. 



The Downhead and Tadhill mass. — In the main, the rocks 

 of the Downhead mass are very uniform in character, and remark- 

 ably like those of Moon's Hill. There is an abundant groundmass, 

 with minute felspar-needles and a considerable amount of glassy 

 matter ; while the phenocrysts include, in addition to plagioclase, 

 both enstatite and augite. The enstatite-crystals are often partly 

 replaced by a yellow serpentinous mineral, and the felspars are, 

 as a rule, in a very advanced state of alteration. The plagioclase- 

 phenocrysts are more numerous in one of the rocks from Downhead 

 Quarry than in any other from the district. 



A section cut from a block found in the field east of the farm 

 called Yellow Marsh, and probably derived from the coarse ashy 

 conglomerate, is of a type somewhat distinct from all the other 

 rocks of the district. It is mainly composed of short square-ended 

 crystals of plagioclase, the length of which is often equal to their 

 breadth. The crystals are all of much the same size, and dearly 

 belong to one generation. They are packed fairly close together, 

 but between them is a certain amount of glass, enclosing felspar- 

 microlites and small serpentinized enstatites in considerable 

 numbers. A few bigger pseudomorphs after enstatite also occur, 

 but there are no large felspars. There are also a few small 

 vesicles filled with a chloritic mineral. 



Sections of the trap, from blocks in the coarse ashy conglomerate 

 west of Tadhill Farm, contain especially large pyroxene-crystals 

 again more or less replaced by bastite or serpentine. 



Summing up the characters of the lava or trap, we find that the 

 rocks show remarkably-little variation in general character, and 

 are all pyroxene-andesites. Small pseudomorphs after rhombic 

 pyroxene are invariably present, and frequently fresh augite in 

 addition. The groundmass is nearly always very prominent, and 

 the felspars occur in the form of microlites, laths, and much- 

 altered phenocrysts of plagioclase, the extinction-angles of which 

 show them to be probably oligoclase or oligoclase-andesine. 



