^Ol* 55*] T^^ ASHBOTJKNE AND BTJXTON KAILWAT. 233 



36 feet is exposed at the southern end of Tissington cutting, about 

 42 feet in Highway Close Barn cutting, and about 96 feet in Crake 

 Low cutting. 



According to the Geological Survey Memoir ' a fault ranges 

 along the south-west side of the valley of Parwich Leys, the lime- 

 stone rising in a bold cliff and dipping away from the shales.'^ 

 The evidence given by the last two cuttings described removes any 

 necessity for the fault by Highway Close Barn and Crake Low, which 

 is marked on the Geological Survey map, Sheet 72 N.E. Though the 

 beds do indeed dip into the hill at Crake Low Quarry, they soon 

 bend over in an anticline, and dip eastward under the shales in the 

 Parwich Leys Valley. 



lY. Peteogeaphy of the Hocks. 

 (1) The thick Ash-bed. 



When freshly cut into, the ash is light-blue or grey, a colour due 

 to the presence of pyrites, but after a short exposure to the air it 

 changes to a rusty red, and later to a greenish-brown. 



The ash is distinctly bedded, and the different beds vary much in 

 texture. The lapilli consist of fragments of a pumiceous rock, from 

 microscopic dimensions up to an inch or so in diameter. The 

 outlines of the lapilli are very irregular, some being more or less 

 rounded, while others, in the more finely laminated portions of the 

 deposit, are elongated in a direction parallel to the bedding, and the 

 vesicles in them are often drawn out in the same direction. The 

 outer surface of the lapillus generally cuts across the vesicles. When 

 fresh, the lapilli are either isotropic or exert only a feeble action on 

 polarized light, and are, therefore, glassy or a devitrified glass. 

 Sometimes they are altered to calcite or dolomite, or to a palagonitic 

 substance, and in one cutting are very much silicified. They are 

 never crystalline. (I have found only one lapillus in which felspar- 

 microlites are present.) There are no small fragments of basalt or 

 dolerite among them, and their structure has no counterpart among 

 the lavas of Derbyshire. 



The vesicles are filled with crystalline calcite, iron-oxide, a light- 

 green fibrous material which exerts only a feeble action on polarized 

 light, and sometimes with an isotropic substance. 



In some cases, as in the ash at the southern end, and in the 

 greater part of that at the centre of Tissington cutting, the lapilli 

 are crowded closely together, and only separated by what is probably 

 a volcanic dust. In others, as in the Highway Close Barn and 

 Crake Low cuttings, where it was possible to obtain less weathered 

 specimens, they are cemented by crystalline calcite. 



Pyrites is largely distributed throughout the ash. The latter has 

 generally a specific gravity of about 2'5 when fresh, but in one 

 specimen of ash which contains an unusually large amount of 

 pyrites the specific gravity is 2*9. 



1 North Derbyshire, 2nd ed. (1887) p. 32. 



