Outlines of Geology. 29 



origin ; they contain the same fossils, echini, sponges, and other 

 substances found in the chalk; and what is curious, many of 

 them are hollow, and contain powdered siliceous earth, provided 

 they have no perforations ; but if hollow and perforated, they are 

 filled with chalk. 



The next beds that occur, in order of succession to the chalk, 

 are several varieties of sand and clay ; the former is often called 

 green sand, from the fragments and particles of chlorite and 

 green earth that it contains, and it is sometimes so compact and 

 hard, as to be fit for a building material of no trivial durability. 

 To what extent this formation accompanies the chalk is not quite 

 obvious, but it occurs in many places on the western side of the 

 chalk range extending from Dorsetshire into Yorkshire, and also 

 upon the coasts of Dorset, Kent, and the Isle of Wight. This sub- 

 stance generally effervesces with acids, from the calcareous matter 

 that it contains, and is abundant in organic remains ; more especi- 

 ally those of the alcyonium, supposed to be a species of zoophite, 

 and seen in a very characteristic manner at the back of the Isle of 

 Wight, where large masses of this rock are lying upon the beach,- 

 having fallen in consequence of the washing away of the marly 

 strata upon which they repose. This marie is of a bluish-black 

 colour — in the Vale of Aylesbury in Bucks, and in that of the 

 White Horse in Wilts, this stratum forms a tenacious clayey soil— 

 and at Shotover Hill, Oxford, it abounds in oyster-shells, selenite, 

 pyrites, and other fossils. Mr. Webster has particularly made 

 us acquainted with the characters and peculiarities of this sub- 

 stance in the Isle of Wight. The sandstone strata that form the 

 perpendicular rocks at the under cliff, lie upon a stratum of blue 

 marie, which being soft and yielding, is occasionally washed 

 away by land springs ; the superincumbent rock of course falls, 

 and to this cause we may attribute that ruined appearance which 

 is so characteristic of the back of the island, and which variegated 

 with woods and corn fields, gives a peculiar and highly picturesque 

 character to that delightful spot. At Black Gang Chine, the 

 washing away of the blue marie by the waters that filter from 

 the higher land, and through the over-lying strata, is particu- 



