664 PRINCIPLES OF STRATIGRAPHY 



tity of fine, argillaceous granules, small, angular fragments of color- 

 less transparent quartz containing numerous minute included cavi- 

 ties, a few similar fragments of flint, siliceous fragments of a 

 glauconite green color, minute crystals of quartz of the ordinary 

 form, minute prisms of tourmaline highly dichroic and similar in 

 form to macroscopic prisms of schorl, and minute rhombohedra of 

 calcite." Materials of organic origin also occur, such as coccoliths 

 and rarely coccospheres, both of the ordinary cyatholith type so 

 common in adjacent seas and in the Atlantic ooze ; further, For- 

 aminifera, including Miliola, Textularia, several species, N onionina 

 crassula, PolystomcUa uinbilicata, Rotalia sp., Spirillina sp., etc., 

 more rarely spicules of Alcyonaria, fragments of echinoderm skele- 

 tons and minute spines ; and triradiate spicules of calcisponges, 

 probably Sycandra. "Most of the Foraminifera are quite empty, 

 glassy and transparent ; but some contain a brownish, soft granu- 

 lar material ; and in one instance a small Rotaline form was ob- 

 served partially replaced by pyrites." (90: 614.) The siliceous or- 

 ganic remains comprise chiefly sponge spicules, very rarely Radi- 

 laria, and a variable quantity of diatoms. The remains of other 

 organisms found in the mud are all of marine types, though they 

 occur on the banks of rivers at a great distance from a truly marine 

 area. 



In the muds of the rivers above the limit of tidal influence, only 

 spicules of fresh-water sponges and diatoms were found {Spongilla 

 fliiviatilis) , foraminifera and other marine organisms being absent. 

 Nor were any of these organisms derived from the Mesozoic 

 strata through which these rivers flow. The organic remains of the 

 modern, as well as the older, muds of the Severn estuary were ap- 

 parently all derived from the Bristol Channel, which is the further 

 enlargement of the Severn estuary and along the shores of which 

 from 10 to 15 miles west of the Severn estuary marine life abounds. 

 The older alluvial deposits of the Severn estuary have a maximum 

 thickness of about 50 feet. These comprise in descending order : 



a More sandy zone 5 to 7 feet 

 Zone I. Upper clay ^ b More argillaceous zone, with disseminated 

 vegetable matter 7 to 8 feet i 

 Upper peat, i to 2 feet 6 inches 



Zone 2. Lower clay 



Lower peat, i to 4 feet 



Zone 3. Sands and mud 

 Gravel 

 Triassic sandstones 



