GEOLOGY 



to view four times. At its second appearance, near the middle of 

 Tissington cutting, the tuff was exposed to view from top to bottom, 

 and proved to be about 140 feet thick. It rests on cherty limestones. 

 It is generally coarse in texture, often has a distinct lamination, and 

 contains numerous blocks, up to one foot in diameter, of a dark blue or 

 amygdaloidal rock, distributed irregularly throughout its mass. The 

 finer portions of the tuff are made up of lapilli varying from one inch in 

 diameter down to a fine dust. They are very vesicular, contain no 

 crystals, and are an altered glass or palagonite. The lower part of the 

 bed consists of small 'ball-like ' lapilli about i inch in diameter. There 

 is an entire absence of non-volcanic material. In the limestones and 

 shales above this thick bed of tuff are numerous intercalations of tuff, 

 some of which are only a quarter of an inch in thickness. After the 

 prolonged eruption or series of eruptions which produced the thick band 

 of tuff, there was a series of intermittent eruptions during the accumula- 

 tion of at least 80 feet of shales and limestones. Volcanic detritus was 

 mingled with the ordinary sediment of the sea bottom, so that a limestone 

 which in one place is entirely free from volcanic material passes into a 

 tufaceous limestone or a shelly tuff in another. 



LAVAS 



The lava flows which were contemporaneous with the formation of 

 the limestone are found on different horizons, and are of limited extent. 

 Some of them were ejected without any accompaniment of tuff. They 

 consist of olivine dolerites and have often undergone a large amount of 

 alteration, due to the action of the weather. When very much weathered 

 they decompose to a kind of clay. They are nearly always vesicular and 

 amygdaloidal, and in places have quite a slaggy structure. In a thick 

 lava stream the central portions are sometimes almost free from vesicles, 

 are hard and compact, and have a spheroidal structure slightly developed 

 in them. 



One of the most typical examples of a lava flow, and also one easy 

 of access, is that seen at the bottom of Miller's Dale just before reaching 

 the station from the south. The beds here form a slight dome, and the 

 river Wye has cut its way down into the Toadstone. The lava contains 

 numerous vesicles and amygdules, and is often slaggy. The base is not seen 

 in the valley, but the junction of the upper surface with the limestone 

 above it may be examined under Ravenstor. The igneous rock has not 

 altered the limestone ; indeed in no case in Derbyshire is a lava flow 

 known to have altered the limestone beds either above or below it. 

 About 150 feet higher in the series is another lava flow, which may be 

 traced from Litton Mills in the railway cutting to Miller's Dale station. 

 It is then carried up by the dip of the rocks and may be traced along the 

 hillsides through the villages of Priestcliffe, Taddington and Chelmerton 

 as far as Great Low on the Ashbourne and Buxton road, where it appar- 

 ently thins out. The same lava flow forms a ring round Critchley Wood 

 Hill, where it is capped by limestone. It forms the summit of Knot Low, 

 i 17 3 



