1850.] AUSTEN SANDS AND GRAVELS OF FARRINGDON. 465 



Dr. Fitton notices a Portland sand as a separable group from the 

 Portland stone at this place ; and as it appears to us, most unneces- 

 sarily : the whole of the lower portion of the mass which succeeds 

 the Kimmeridge clay is arenaceous, of which the building-stone 

 constitutes subordinate bands. After an accumulation of about five 

 feet of sands had taken place, the conditions of sea-bed favoured 

 the development of a great bank of the Ostrea falcata, Sow., and 

 Ferna quadrata, Sow. ; the bed is from one to two feet thick, com- 

 posed to a great extent by these two forms alone, with fine sedi- 

 mentary matter in layers. Sands, now mostly passed to the condi- 

 tion of sandstone, succeeded, and which for the first few feet are re- 

 markably rich in the usual fossils of the Portland {Trigonia clavellata, 

 Park., Cardium dissimile, Sow., which here attains a great size). 

 Throughout the remaining portion of the sands and sandstones fossil 

 remains are comparatively scarce, with the exception of Ammonites^ 

 which occur irregularly. The grain or line of deposition of the 

 building-stone masses accords with the bedding, whilst the arrange- 

 ment of the intervening sands is mostly diagonal. The particles 

 composing the stone are finer than those of the sands, and they con- 

 tain a proportion of lime ; the diiference is due in the first instance 

 to a varying moving power of the water. 



In the calcareous beds of the Swindon Portland, organic remains 

 again become abundant, and certain new forms predominate, such as 

 the Terehra Portlandica, Sow., which alone forms a band of some 

 thickness near the base of the pure limestone portion. 



The limestone mass, when its lower portion is compared with its 

 upper, presents a slight mineral change, which hardly admits of being 

 described, but which is perceptible on the spot — and this change is 

 found to correspond with the condition of the mass of water under 

 which it was deposited — the lowest portion is marine, the forms in 

 the upper are those of fresh water. The only feature which at all 

 indicates a change of condition of the water is the gradual decrease 

 of the forms so abundant in the lower portion of the limestone ; there 

 is no passage through brackish water forms, nor any break to show 

 an interval of time, or a change from sea-bed to that of a lake, but 

 simply such a change as would be the result of the influx of a mass 

 of fresh water into the oolitic sea, bringing with it its own peculiar 

 forms, and thus rendering a given area objectionable to marine ones. 

 The diminished size of the Cardium may perhaps be taken as an in- 

 dication of such a change in the condition of the water, and the small 

 Ostrece which occur within the mass containing Cyclas 1 and Cypris 

 would clearly show that the calcareous beds still belonged to the 

 area of the oolitic sea. 



At a time subsequent to the first indications of this change of con- 

 dition the limestone mass has been greatly disturbed, great blocks 

 have been detached, and rolled about on the surface of the mass, and 

 these are often eaten out by some excavating animal, and we must 

 consider these fragmentary beds as having been produced by the 

 ordinary action of the sea, when the mass had been brought up to 

 the condition of a submerged reef, and within the reach of wave-ac- 



