138 PKOF. T. T. GROOM ON THE GEOLOGICAL [May 1 899, 



the broken character of the more southerly strip, it is highly probable 

 that a transverse fault separates the two series, and this view fits in 

 better with the theoretical explanation of the structure of the hill 

 given on p. 153. 



The pentagonal block forming the north-western corner of 

 Raggedstone Hill is crossed by a thick dyke of andesitic basalt. 

 This, being a durable rock, stands out, and forms the western face of 

 a prominent rib running down from near the summit of the hill to 

 cross the road in the Hollybush Pass. It is not an interbedded 

 lava, as supposed by Holl,^ but is clearly intrusive, for it crosses the 

 outcrop of the sandstone-beds in a marked manner, as shown in 

 fig. 1 (p. 132). Where it is intersected by the road, it is seen to 

 dip westward at an angle of about 30°. 



Along its western boundary, the Hollybush Sandstone is succeeded 

 for most of the length of the hill by the Black Shales. The junction 

 with these also is a fault ; for, although at one spot the Hollybush 

 Sandstone has a reversed north-easterly dip, elsewhere along the 

 line of junction the dip is south-westerly, while that of the Black 

 Shales, wherever they are clearly exposed, is north-easterly. 



(2) Midsummer and Hollybush Hills. (Figs. 8-10.) 



The portion of the range formed by these twin hills shows, in 

 some respects, a structure similar to that of the liaggedstone. The 

 joint mass is quadrangular in shape, and the two hills are separated 

 on the south by a deep depression in which were formerly situated 

 an old British town and what were apparently three old British 

 reservoirs (PI. XV). The northern portion of the mass is divided 

 into three parts by two depressions, one of which, running east- 

 north-eastward, forms the northern boundary of Hollybush HiU ; 

 the other runs northward. 



The bulk of the two hills is composed of a mass of gneissic, 

 schistose, and massive rocks, probably traversed by a number of 

 faults, as indicated by sudden variations in the strike of the folia. 

 On the whole, the mass exhibits a markedly plagioclinal structure, 

 the foliation usually showing no relation to the boundaries, a 

 feature in which these hills differ from Eaggedstone and Chase 

 End Hills. But in the large quarry at the southern end of HoUy- 

 bush Hill the schists dip east-south-eastward at a high angle. 



The Archaean Series is bounded on the east by the Trias, along 

 a line of fault. The breccia marking this fault-line, and consisting 

 of fragments of Archaean rocks and Triassic sandstone, is exposed in 

 an old quarry on the roadside (M 191)." To the north a transverse 

 fault brings the gneissic series against that of Swinyard Hill, with 

 its flanking deposits of May Hill Sandstone. The lower part of 

 the western slope of Midsummer Hill, for the greater portion of its- 



^ Quart. Journ. Geol. Soc. vol. xxi (1865) p. 87. 



^ [Omitted unintentionally from the map, PI. XIII.] 



