234 MR. H. H. AENOLD-BEMEOSE ON THE GEOLOGY OF [May 1 899, 



The lower part of the thick ash-bed is seen only in Tissington 

 cutting. It is made up of small pieces of a vesicular rock about 

 1 inch in diameter, closely packed together. These are spherical or 

 discoidal in shape, and when weathered are spotted with carbonate 

 of lime, which at a short distance gives the rock the appearance 

 of an amygdaloidal lava. They are not bombs, but large lapilli, 

 which have probably derived their present shape from having been 

 nibbed together in the volcanic vent, prior to their final ejection. 



A thin slice from one of these spheroids, |- inch in diameter, was 

 examined under the microscope. During the process of grinding it 

 was broken up into small pieces, which looked like separate lapilli ; 

 but on further examination it was seen that the outlines of any two 

 in juxtaposition correspond in such wise that if placed close together 

 they would fit exactly. The structure is very similar to that of the 

 minute lapilli described above. Some of the vesicles are circular in 

 section. Others are very much elongated, but their longer axes bear 

 no relation to the outer boundary of the fragment. On the contrary, 

 the boundary breaks across the vesicles, as in the case of the smaller 

 lapilli, and also in that of the larger included blocks described 

 below. 



In Crake Low cutting some parts of the ash-bed are very hard. 

 Both lapilli and matrix have been altered to silica, calcite, and 

 pyrites : the various stages in this alteration can be traced. A 

 fairly fresh specimen is composed of isotropic lapilli in a calcific 

 matrix. Cryptocrystalline silica then appears in small patches 

 the lapilli are next altered to calcite, and embedded in calcite and 

 cryptocrystalline silica with pyrites ; and, in the final stage of 

 alteration, the rock is entirely composed of the above-mentioned 

 minerals, the outlines and vesicular structure of the lapilli being 

 visible in ordinary light. 



In the upper part of the thick ash, at the northern end of 

 Highway Close Barn cutting, several beds of a fine-grained and 

 laminated mudstone were seen. The specific gravity of two 

 specimens was 2'32 and 2-4, which is less than that of the ash. 

 One had the appearance of a dull, fine-grained basalt, though it was 

 not crystalline : the other was softer and less coherent. Under the 

 microscope they are almost structureless, and contain iron-oxide, 

 with a small quantity of crystalline calcite, but no traces of lapilli. 

 There is no microscopic evidence of their volcanic origin. 



(2) The Ejected Blocks in the thick Ash. 



The ejected blocks found m the ash are generally rounded or 

 subangular in shape, and either vesicular or amygdaloidal. When 

 the vesicles are filled with a dark material, the rock looks (on a 

 freshlj^-fractured surface) very much like limestone. The blocks vary 

 in size from several inches up to about a foot in diameter, and are not 

 arranged in beds, but scattered irregularly through the finer ash. 

 They difi'er from t'le small fragments in the ash by being finely 



