H. Cooksey Burton — Yellow Sands and Marl. 301 



1. The small pockets and hollows are of various dimensions, having 

 a diameter from 2 to 5 inches ; these pockets form a network 

 simulating an irregular honeycomb structure. In these small pockets 

 the sand and marl are irregularly mixed together, so that no regular 

 bedding can be distinguished. 



2. The second type is shown in Fig. 3, where highly inclined 

 parallel fissures are connected with the main horizontal fissure. The 

 included beds consist mainly of impure 'Yellow Sands', becoming- 

 more marly towards the ends. The bedding is conspicuous and 

 conforms very noticeably to the irregular roof. 



3. Fig. 4 shows a section of the thickest and most extensive of 

 the lenticular beds of ' Yellow Sands ' ; its total thickness is 4 feet 

 and its length slightly more than 20 feet ; it is remarkable as showing 

 alternations of sand and marl in well-bedded horizontal layers and in 

 presenting a well-marked case of false bedding in the sand (see A, 

 Fig. 4). The marl is quite moist and soft and contains a considerable 

 amount of common salt. The water pumped up from considerable 

 depths in the Permian is generally salty, and its specific gravity 

 sometimes approaches that of sea-water. 



The area over which these peculiar included beds of ' Yellow 

 Sands ' are scattered is fairly extensive, and the fact that both in the 

 north-south and east-west sections their shape is lenticular is an 

 argument in favour of their occupying cavern-shaped hollows in the 

 breccia. More than a dozen of these pockets filled with sand and 

 marl are scattered over the Trow Rocks area. 



The ' Yellow Sands ' as they occur in situ beneath the Marl Slate 

 consist of well-polished and rounded grains of quartz coated with 

 oxides of iron, which give to the sand various colours from light 

 yellow to dark brownish-red according to the percentage and state of 

 hydration of the iron oxide. The sand from the above pockets and 

 lenticular beds was identified with certainty as true ' Yellow Sands ' 

 not only by the rounded nature of the grains, but also by a comparison 

 of the percentage of its heavy minerals with that peculiar to the 

 ' Yellow Sands ' occurring in their natural position at Frenchman's 

 Bay, half a mile to the south. 



The ' Yellow Sands ' all along their outcrop contain varying 

 percentages of heavy minerals having a specific gravity greater than 

 2'80 ; these were determined by the use of Sonstadt solution in 

 Sollas's separating funnel. At Frenchman's Bay the percentage of 

 heavy residue is - 17 per cent., while in the Trow Bocks ' Yellow 

 Sands' it is - 2 per cent.; in each case the heavy minerals are 

 similar, being mainly garnet, tourmaline, zircon, rutile, and ilmenite, 

 with leucoxene. 



It is thus established that the sand occurring in the pockets in 

 Magnesian Limestone at Trow Bocks is identical with the bed of 'Yellow 

 Sands ' which occurs in place at Frenchman's Bay between tide- 

 marks. The light-yellow marl is a mixture of calcium and magnesium 

 carbonates containing clayey matter in varying quantities. So far as 

 our knowledge extends at present the ' Yellow Sands ' are invariably 

 accompanied by the Marl Slate when in their natural position, being 

 directly under this group of beds, which occurs at the base of the 



