Part of Gloucestershire and SomersetsJiire. 323 



§ 7. Before we enter upon the consideration of the transition and car: 

 boniferous series, it may be useful to observe that in both are to be found 

 sandstone, slate-clay, clay, marl, and limestone, the former including also 

 trap. The sandstone and limestone of the transition and carboniferous series 

 frequently resemble each other so nearly as not to be easily distinguished by 

 mere mineralogical character : but their different position in the order of 

 succession, the abundance and variety of peculiar organic remains in the 

 transition sandstone and limestone, and the absence of such remains in the 

 carboniferous series in general, and especially in the old red sandstone, suf- 

 ficiently distinguish the two series from each other. 



I. Transition Tract*. 



§ 8. 1 propose to trace the transition beds from Pyrton to the vicinity of 

 Stone, and thence to explore the valleys of the Avon and Falfield. But, as a 

 preliminary, let us consider the leading characters of the members com- 

 posing this transition tract. 



The constituents of the transition sandstone are grains of quartz or sand, 

 generally very fine, and sometimes so minute as to be almost imperceptible, 

 among which are scattered minute scales of silvery mica that are often scarcely 

 visible, except when examined by the lens. Particles of felspar are very 

 rarely to be observed. The mass is coloured reddish, or greenish gray, of 

 various shades, both separately and intermingled in a mottled manner, ac- 

 cording; to the variable state of the oxidation of the iron that enters into the 

 composition, or the proportion of the matter of clay, or slate-clay, that is 

 casually intermixed. Sometimes also the sandstone is coloured greenish by 

 chlorite, diffused through its substance ; but in general the reddish hue pre- 

 dominates : while the state of aggregation varies from the most firm to that 

 which is sandy, or earthy, and perishable ; the compound, however, being 

 commonly very compact and tenacious. In some cases also the particles are so 

 closely approximated as to constitute a quartzy sandstone, or quartz rock, 

 passing into compact quartz, or hornstone, of a gray or white colour. When 

 adjacent to limestone or slate-clay, the sandstone is frequently intermingled 

 with those minerals, and contains discontinuous layers and portions of them 

 interposed between the strata. 



The slate-clay is usually of a reddish, or greenish gray cast, earthy, soft, 

 and perishable ; and contains more or less of mica, scales of which appear in 



* The term transition was well chosen by Werner, as designating a class of rocks the earlier 

 members of which approximate, in mineralogical character, more and more to the primary, — and 

 the later, to the sedimentary formations ; — while at the same time, containing the first vestiges 

 of organized bodies, the class comprehends the oldest of the secondary rocks. 



