ME. H. H. AENOLD-BEME.OSE ON [Aug. I907,. 



is abrupt, and the faces of the former rock left bare by the denu- 

 dation of the agglomerate represent as nearly as possible the walls 

 of the vent. 



The agglomerate may be traced across the valley, and for a 

 slight distance up the western slope, where the adjacent limestones 

 are nearly horizontal. On the east the limestones, at least 100 feet 

 higher in the series than those in the bottom of the valley, are 

 seen within a few feet of the agglomerate and have a slight dip to 

 the east. 



A description of the microscopical structure of the agglomerate 

 is given in Quart. Journ. Geol. Soc. vol. 1 (1894) p. 632. 



Grangemill Yents. — The Grangemill Yents, with their 

 associated band of tuff about 90 feet thick, form perhaps the most 

 interesting and at the same time the most accessible volcanic group 

 in the county. They have been described and figured by Sir 

 Archibald Geikie 1 and by me. 2 With a view of making this paper 

 complete, it will therefore suffice to give a brief summary of the 

 results, with the addition of further investigations that I have made. 

 About 5 miles from Matlock Bath, beyond the top of the Yia Gellia, 

 two dome-shaped hills with grassy slopes and well-marked contours 

 present a decided contrast to the limestone-scenery which surrounds 

 them. Their summits rise to a height of 200 and 100 feet 

 respectively above the small valley in which they are situated. 

 Good views of them may be obtained from the higher ground 

 between Grangemill and Aldwark. The best exposure of the 

 agglomerate is seen in the larger vent close to Grangemill on the 

 road to Winster. A glance at the map (PI. XXI) shows that the 

 vents are in the northern part of a dome of limestone, the longer 

 axis of which runs in a north-north-westerly direction. Imme- 

 diately south-west of the smaller vent the limestone dips 

 north-north-westward, at an angle of 10° to 15°. South of the 

 larger one the limestone is much disturbed, and in places is 

 horizontal and strikes at the agglomerate. The agglomerate there- 

 fore cuts across the strike in both cases. The rock in the vents 3 

 consists of an agglomerate of lapilli in a cement of volcanic detritus 

 and a small quantity of calcite, with blocks of more or less marmorized 

 limestone. A few years ago I found in the larger vent an agglo- 

 merate similar to that seen in the Ember-Lane Yent (pp. 263, 264). 

 It consists of lapilli with small pieces of limestone, embedded in a 

 cement of limestone and limestone-paste. 



Three small dykes penetrate the larger vent, and run from west 

 to east. Sections of them are to be seen on the roadside opposite 

 the old Mill. The northernmost dyke is several feet wide, and I 

 have been able to trace its presence about 400 feet to the west of 

 this point. The two other dykes are much smaller, and one of 



1 ' Ancient Volcanoes of Great Britain ' vol. ii (1897) pp. 13-15. 



2 ' Sketch of the Geology of the Lower Carboniferous Bocks of Derbyshire ' 

 Broc. G-eol. Assoc, vol. xvi (1899-1900) pp. 198-200. 



3 See Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 633-35. 



