134 PROCEEBINGS OF THE GEOLOGICAL SOCIETY. 



mentary material connected with the pu3's. They are usually dull 

 green in colour and granular in texture, the lapilli consisting of 

 various decayed basalts in a soft paste of the same materials. Many 

 of the lapilli are highly vesicular — a kind of basic pumice. Among 

 the bedded tuffs, stratification is generally well-marked by alter- 

 nations of finer and coarser material. Occasional large blocks or 

 bombs indicating some paroxysm of explosion may be observed 

 even among the finer tuffs, shales, and other strata, which round 

 the sides of these masses have had their layers bent down by 

 the fall of heavy blocks.^ Interstratifications of non-volcanic 

 materials are of constant occurrence among the tuffs, which con- 

 sequently pass both laterally and vertically into sandstone, shale, 

 limestone, &c. Many of the bedded tuffs likewise contain fossils, 

 such as crinoids, corals, brachiopods, fish-teeth, or macerated frag- 

 ments of land-plants. Coal-seams also are occasionally interstratified 

 among them. 



The materials which fill the necks are generally much coarser 

 than those that form intercalated beds. Huge blocks of basalt and 

 large masses of sandstone, shale, limestone, ironstone, or other strata 

 may be seen wrapped up in a matrix of coarse basalt-tuff. But 

 many necks may be observed to consist of a tuff quite as fine as 

 that of the beds. Such necks appear to mark the sites of tuff- 

 cones where only fine ashes and lapilli were ejected, and where, 

 after sometimes a brief and feeble period of activity, the orifice has 

 become extinct. 



III. Geological Steuctuee. — The puy-type of volcanic hill 

 differs widely in one respect from those which we have hitherto 

 been considering. In the earlier epochs of vulcanism within the 

 British area, it is the masses of material discharged from the vent 

 rather than the vents themselves which arrest attention. Indeed, 

 so copiously have these masses been erupted that the vents are often 

 buried, or their positions have been rendered doubtful, by the uprise 

 in and around them of sills and bosses of molten rock. But among 

 the Carboniferous puys the vent is often the only record that 

 remains of the volcanic activity. In some cases we know that it 

 never ejected any igneous material to the surface. In others, 

 though it may be filled with volcanic agglomerate or tuff, there is no 

 record of any shower of such detritus having been discharged from 

 it. In yet a third class of examples, we see that lava rose in the 

 vent, but no evidence remains as to whether or not it ever flowed 

 1 Geol. Mag. for July 1864, p. 22. 



