DOLOMITIC CONGLOMERATE NEAR TORTWORTH. 



451 



described by several geologists 1 , and in this tract especially by Mr. Weaver, whose account of their 

 structure is so good that I cannot do better than quote it. " They are composed principally of 

 rounded and angular fragments of limestone and sandstone, sometimes exceeding the size of the 

 head, with fragments also of hornstone and quartz : these are cemented by a calcareous paste, which 

 is frequently of a marly nature, or a common carbonate of lime, either of an earthy or compact 

 structure ; but in some quarters, as in the Vale of Thornbury, the cement is generally magnesian, 

 and through all the varieties sandy particles are more or less distributed. The compound abounds 

 in cavities which are frequently lined with crystals of calcareous spar and quartz, and sometimes 

 also with sulphate of strontian." 



The exact range of the conglomerate and magnesian limestone is given in the accompanying map. 

 It generally forms an irregular broken fringe, hanging upon the flanks of the older rocks, sometimes 

 consisting of a very coarse conglomerate of considerable thickness, which rests either on the car- 

 boniferous limestone or Old Red Sandstone, the fragments in each case being made up respectively 

 of the detritus of these rocks. On the sides of the new Bristol road, as it descends to Thornbury, 

 the following section of the passage of this conglomerate into limestone has been recently exposed. 



a. Yellow sandy beds, covered by red detritus, b. Impure limestone, about one foot tbick. c. Courses of limestone, 

 two to tbree inches thick, alternating with yellow sands, d. Yellow sandstone, partly calcareous, e. Dolomitie conglo- 

 merate resting on carboniferous limestone, and made up of large fragments of that rock. 



To the east of Thornbury the dolomitie conglomerate rests directly on the coarse conglomerate 

 of the Old Red Sandstone, the beds of the older being almost as horizontal as those of the newer 

 conglomerate. It is again seen at the same inclination and in conformable apposition to flaglike 

 beds of the Old Red Sandstone to the south of Kington. The reader wishing to acquire more co- 

 pious information respecting this conglomerate, as it expands into the districts around Bristol and in 

 the Mendip Hills, must consult the various memoirs in the Geological Transactions in which it is 

 described. My principal object is to direct attention to the different relation of the conglomerate to 

 the older rocks in the south-west of England, and in Shropshire, Staffordshire, and Worcestershire. 

 In the former district, it has just been shown to lie unconformably upon the coal measures and more 

 ancient strata ; while in the latter it was proved in previous chapters to pass down gradually into 

 the true coal measures, through the Lower New Red Sandstone, which being almost entirely absent 

 in this region, accounts to us for the unconformability between these two deposits. 



It is, therefore, essential to remind the reader that the great break marked by 

 Messrs. Buckland and Conybeare at the base of the dolomitie conglomerate, and which 

 is so extensively and clearly displayed in the south-west of England, must be considered 

 as only a partial phenomenon ; for it has been clearly ascertained, that the same action 

 which there broke off the connecting links between the New Red System and the coal 

 measures, did not extend into the central counties ; on the contrary, we have seen that 

 all the intermediate strata have been there deposited in regular and unbroken succes- 

 sion. (See p. 46 et seq. and the following chapter.) 



1 Geol. Trans., Old Series, vol. iy.j in memoirs by Dr. Knight, Mr, Warburton, and Mr. Gilby, and also 

 by Messrs. Buckland and Conybeare in the memoir before mentioned. 



