Page:VCH Derbyshire 1.djvu/65

 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