MATERIALS 17 



(b) Geological 



Possession of durable structures, and selection of a 

 locality favourable for burial, in no sense ensure fossiliza- 

 tion. The nature of the materials in which shells are 

 embedded, and the geological influences that may 

 operate on them, must inevitably affect the preservation 

 of fossils. The three types of sedimentary matrix which 

 most frequently contain fossils are those known as 

 arenaceous (sandy or gritty), argillaceous (clayey), and 

 calcareous (ooze or shell-sand). When such deposits 

 are formed under marine conditions, it is usual for the 

 first-named type to occur on or near the beach, for the 

 second to accumulate in the outer part of the detrital 

 zone, and for the third to be spread over the shallower 

 parts of the open ocean. Some calcareous rocks, such 

 as oolites and shell-limestones, are developed in the 

 position normally occupied by sand. Although no 

 definite line can be drawn between the three types of 

 sediment, owing to the frequent intermingling of their 

 ingredients, when relatively pure they afford very 

 different matrices. 



Sand is largely composed of worn grains of quartz, 

 the hardest and most durable form of silica, and is 

 likely to enclose abundant remains of shell-bearing 

 Invertebrates owing to its littoral distribution. But it 

 is eminently porous, and percolating water, which cannot 

 attack quartz, will find in the calcareous shells material 

 relatively easy to dissolve. Sand-burial thus affords 

 but poor protection from the weather. The enclosed 

 remains may occasionally become petrified by replace- 

 ment, if the water that dissolves them holds some less 

 soluble salts in solution; but more often they are 

 entirely removed. If the arenaceous rock has been 

 compacted before the dissolution of the shells, casts and 



