230 ME. H. H. THOMAS OX THE [Aug. 1909, 



ofteu loose and incoherent, but occasionally compacted into rocks 

 of more solid character. 



The Lower Sandstones, which for the most part occupy a position 

 above the breccias, have been studied on each side of the Exe 

 estuary, in the vale of the Clyst, in the neighbourhood of Killerton 

 and Brampford Speke, and in the Bridgwater area. They present 

 many points of similarity to the sands and sandstones interbedded 

 with the lower breccias. 



The Lower Marls, consisting of red marls with sandstone-courses, 

 the Pebble-Bed, and the Upper Marls and Sandstones have been 

 dealt with from numerous localities in the two counties. 



At first I confined my attention to that part of the country which 

 lies between Wiveliscombe on the north, and the south coast of 

 Devon ; but later, at the instigation of Mr. Ussher, I extended the 

 work so as to include the red rocks lying to the west and east of 

 the Quantock inlier, in the districts of Taunton and Bridgwater. 1 



Nearly one hundred samples were collected from various localities 

 and horizons, each sample receiving the following treatment : — The 

 sand or crushed sandstone was first put through a fairly fine sieve, 

 so as to separate out the larger rock-fragments and leave, as far as 

 possible, simple mineral grains. The coating of iron-oxides was 

 then removed from the grains forming the finer portion, which, 

 after washing and drying, was subjected to two fractional separa- 

 tions by means of Thoulet's solution. 2 Three residues were thus 

 prepared from each sample, of which portions were mounted for 

 microscopical study. One residue contained all those minerals 

 present in the sample which had a specific gravity greater than 

 2*8, the other two respectively contained the bulk of the quartzose 

 and of the felspathic material. The heaviest residue is in general a 

 very small percentage of the whole sample, but at the same time the 

 ratio of the lightest part to the heavier varies considerably in all 

 divisions of the New Bed Sandstone. 



In some respects, the treatment of sands and sandstones with 

 hydrochloric acid is disadvantageous : for, while the iron-oxide, 

 which coats the grains, is removed, it is probable that some of the 

 less stable minerals are attacked and even eliminated. If, however, 

 after such treatment mineralogical differences can be detected in 

 the residues from various samples, such distinctions are obviously the 

 most reliable : for, if our considerations extended to minerals easily 

 attacked by acids, we should then deal with minerals which might 

 be removed, locally from the deposit by natural chemical agencies, 

 and this would give misleading ideas as to their distribution. 



1 ' Geology of the Quantock Hills, &c.' Mem. Geol. Surv. 1908, pp. 60-64. 



2 A solution of the double iodide of mercury and potassium in water. 

 Although it has a great drawback in its poisonous nature, I have found it 

 by far the cleanest to handle and the easiest of recovery. It has many 

 advantages over other heavy liquids; it is less viscous than a solution of 

 cadmium-borotungstate of the same density, and is not so prone to develop 

 convection-currents with slight changes of temperature as in the case of the 

 more mobile organic fluids (bromoform, etc.). 



