Vol. 55.] THE ASHBOFRKE A^D BUXTOIT RAILWAY. 235 



crystalline, and not so glassy. They are similar to the blocks in the 

 Kniveton vent, the structure of which was described by me in a 

 former paper read before this Society,^ but the felspars are much 

 smaller, and there are very few traces of olivine. Their specific 

 gravity varies from 2-53 to 2-62. 



Two blocks found in Highway Close Barn cutting are probably 

 ejected fragments of the older ash which solidified in the vent, and 

 were blown out by subsequent eruptions. They consist of isotropic 

 and well-preserved lapilli in a cement of cloudy calcite. 



(3) The thin TufiPs and Tufaceons Limestones above 

 the thick Ash. 



Among the thin limestones and shales which lie above the thick 

 ash-bed are numerous intercalations of tufi", varying in thickness 

 from about | inch to 2 feet. They are very similar to the thick 

 ash just described, but diff'er from it in containing Productus and 

 large encrinite-stems : they weather rapidly, and are generally too 

 incoherent for slicing. 



A bed of coarse tuff (2 feet thick) in Crake Low Quarry, and 

 containing Productus semireticulatus and an ejected block, proved 

 sufficiently hard for a thin slice to be prepared. It is light-brown, 

 with light-grey patches, in a hand-specimen. Under the micro- 

 scope, it consists of numerous lapilli, altered to crystalline calcite, 

 unevenly distributed throughout the slice, and mingled with small 

 pieces of previously consolidated limestone, which often contain a 

 foraminifer or a portion of one, or of some other organism. All 

 these fragments are cemented together with crystalline calcite. 

 The vesicular block, several inches in diameter, found in this bed is 

 similar to those from the thick ash in the cuttings. 



In the thick ash-bed there appears to be an absence of ordinary 

 mechanical sediment, while in the limestones and thin partings of 

 tuff above this bed there is a commingling of volcanic and ordinary 

 sediment in rapidly varjdng degrees, even on the same horizon, 

 within a short distance. Some of the limestone-beds are free from 

 tuff, and others contain varying proportions of volcanic ejectamenta ; 

 and thus what is a limestone free from volcanic sediment in one 

 place will pass into a tufaceous limestone or a shelly tuff in another. 

 Some beds, which in a hand-specimen resemble a hard tuff, when 

 examined under the microscope are seen to contain so many fossils 

 that they may be considered as a tufaceous limestone. 



The lapilli distributed through the limestones are almost invariably 

 altered to crystalline calcite or dolomite and oxide of iron. Only 

 one small lapillus, in a limestone 20 feet above the ash in the 

 northern part of Tissington cutting, contained felspar-microlites. 

 When altered to calcite or dolomite they are hard to distinguish 



^ ' Microsc. Struct, of Carb. Dolerites & Tuffs of Derbyshire,' Quart. Journ. 

 Gaol. Soc. vol. 1 (1894) pp. 638, 639. 



