236 ME. H. H. AENOLD-BEMROSE ON THE GEOLOGY OF [Maj 1 899, 



in polarized light, and their vesicular structure is seen with such 

 difficulty in ordinary light that they might be easily overlooked. 



The limestones in which they are embedded generally contain 

 foraminifera, shell-fragments, and small encrinite-stems. In many 

 cases the tufaceous limestones consist (in addition to volcanic 

 material) of worn shell-fragments and small pieces of a previously 

 consolidated limestone, which sometimes contain a few quartz- 

 crystals. Patches of cryptocrystalline silica are present in some 

 specimens. Parts of a limestone-bed are often dolomitized. 



In a few cases the lapilli are isotropic, as, for example, in a 

 tufaceous limestone resting upon the top of the ash at the centre of 

 Tissington cutting, in a tuif (10 inches thick) immediately above 

 it, and also in a tuff-parting (6 to 8 inches thick) 2 feet above the 

 thick ash in Highway Close Barn cutting and separated from it by 

 shales. 



The tufaceous limestones in Crake Low Quarry are similar in every 

 respect to those in the cuttings. About 28 feet from the top of the 

 quarry is an interesting bed of banded limestone, which shows a 

 series of rapid alternations of ordinary and volcanic sediment. 

 Above and below it are 2 and 3 inches of tuff respectively, which 

 die out in a distance of 6 feet to the south, and are replaced by 

 limestone. 



In a hand-specimen nine black bands occur in a space of less 

 than 1 inch, then follows |inch of limestone free from bands, and 

 lastly seven more bands in the space of an inch. Under the 

 microscope the bands consist of elongated lapilli, coloured with 

 iron-oxide : these are isotropic, and often are penetrated by and 

 contain small rhombohedra of dolomite. 



(4) The Limestones. 



The limestones interbedded with the shales vary considerably in 

 structure and texture, and often contain layers of chert. Some of 

 them are coarse in texture and composed mainly of encrinite-stems 

 and Productus ; others are dai-k, fine-grained, and free from fossils. 

 Many of them contain foraminifera. The coarse and fine-grained 

 varieties are often found in different layers of the same bed. Many of 

 the limestone-beds are partly or wholly dolomitized, and in extreme 

 cases all traces of fossils are obliterated. When the dolomitized 

 beds are weathered they have a sandy fircl and appearance, and on 

 a cursory examination might be mistaken for a sandstone. They 

 sometimes contain cryptocrystalline silica. 



Two thin slices of the compact limestone below the ash-bed in 

 the centre of Tissington cutting were examined. The rock is 

 a very close-grained and partly crystalline limestone, with a few 

 small encrinite-stems. 



Altogether, forty-six thin slices were examined from the rocks in 

 the three cuttings and Crake Low Quarry. 



