XVI.-- Geological Observations on Part of Gloucestershire and Somersetshire
Transactions of the Geological Society of London
Pyrton on the north; the western side being formed by the left bank of the Severn, and the eastern by a line drawn from Pyrton along the great escarpment of oolite. Of the tract included in this area, I have represented in the annexed map, only so much as, when combined with the sections by which it is accompanied, may be sufficient to convey a clear idea of the relative position and order of succession of the different formations. (See the Map, Plate XXXIX., and Sections No. 1. 2. and 3.; of
... 1. 2. and 3.; of which the Section No. 3. occupies the whole base line of the triangle*). The structure of the north-west of Gloucestershire is illustrated by a section drawn from Huntley-hill on the north-east, to the Forest coal-basin on the south-west (see Plate XXXIX., Section No. 4.); and that of the north-west of Somersetshire, by one general section (No. 5.), drawn from the Avon at Bristol on the north, to the river Parret at Bridgewater on the south ; and by a second section (No. 6.), traversing Mendip only, in the vicinity of Shipham. ENVIRONS OF TORTWORTH. General View.--Secondary Formations. § 4. One of the first impressions made on the mind, in examining the Tortworth district, is, that the existing form of the surface appears, to a certain extent, to be unconnected with the nature of the rocky formations that compose its base ; an observation, indeed, that may admit almost of universal application, and be deemed a maxim in geology. A second impression is, that if the distribution of formations into Primary, Transition, and Flcetzf or sedimentary classes, be well founded,-as resting on distinct epochs in the history and structure of our planet, the evidence of which is to be discovered in their respective characters and relations; it is also true, that natural distinctions point to a subdivision of the sedimentary formations, into collective series or groups, consequent to, and in a great measure independent of each other, with respect to the era of their production. Under this view, the first series of the sedimentary class, namely, the carboniferous, comprehends, in the * Having no map of Gloucestershire, that conveys an adequate idea of the form of the surface, I constructed the annexed plan and sections of the environs of Tortworth from a survey, in which my pocket-compass and clinometer were the only instruments employed. I trust, however, that they will be found to represent faithfully the form of the country. + The use of the German term Jloetz is no doubt so far objectionable, as it does not harmonize with our language : but in its full comprehension the word was intended to convey a complex idea, for the expression of which we have not any corresponding English term, and to denote formations whose leading characters are, a distinctly stratified structure, combined with a YOL. VI. 2 T at University of Colorado Boulder on December 17, 2016 http://trn.lyellcollection.org/ Downloaded from 320 Mr. WEAVER'S Geological Observations on vicinity of Tortworth, the old red sandstone, carboniferous limestone, and coal formation ; the second (or gypseous and saliferous series) includes the calcareous conglomerate, magnesian limestone, and new red sandstone; of the third series, which comprises the lias and oolite limestones, the iron and green sandstone, and the chalk, the lias and oolite limestones only appear within this tract: and the fourth and last series, or the formations later than the chalk, are also wanting. The first, or carboniferous series, reposes upon a transition base, while the succeeding formations overlie, unconformably, both the carboniferous group and the transition tract; and this either continuously or in dismembered portions. § 5. In the parallel of Wickwar, (Section No. 3.), the elevated ridge of the old red sandstone and its accompanying limestone, the strata of which dip here toward the west, supports, high up on its eastern flank, a deposition of calcareo-magnesian conglomerate, that upholds red clay marl (of the new red sandstone formation) sustaining lias; which last extends to the foot of the oolite escarpment, beyond Ingatestone Common, on the east, a distance of near three miles :-all these beds being arranged in a nearly horizontal position, and forming stages more or less in the form of table lands, with a dip of 2° or 3° towards the east. This part of the course of the western branch of the Avon, is wrought out of the new red clay marl, and the subjacent calcareomagnesian conglomerate ; but in descending to the north, the valley expands composition evidently and principally resulting from mechanical agency. I do not however see any material objection to the adoption of the term sedimentary, as a substitute for Jlcetz; and hence I shall employ it throughout the remainder of this paper. That sedimentary deposits occur in transition and primary tracts also, can be no valid objection to the use of the word in this more large and characteristic sense ; for these deposits constitute in such cases merely partial and local sub-formations. In the same manner the term old red sandstone is not incorrectly applied to the first member of the carboniferous series, although red sandstone occurs also in transition tracts, and, it is affirmed, even in the primary; the latter cases being only exceptions to a general and opposite rule, namely, that a crystalline structure is the more prevalent characteristic ; while the red sandstone appears as a consistent and predominant formation, for the first time, and is therefore in a geological sense the oldest, in the first great sedimentary or carboniferous period.