2 MOLLUSCA FROM THE GREAT OOLITE. 



The mineral masses which constitute this series of beds are exclusively of marine origin, 

 the varying character of their organic contents being connected both with the mineral 

 character of the deposit spread upon the floor of the ancient sea, and with its depth. 

 These deposits may be conveniently divided into three groups : 



1st. The Weatherstones ; 3d. The Sandstones ; and 3d. The Limestones. 



The weatherstones, which are situated at the base of the formation, average about 

 40 feet in thickness. They consist of shelly sandstones, abounding with crystalline 

 carbonate of lime, and having Oolitic grains irregularly and sparingly distributed throughout 

 their mass. The variety of mineral character is so great, that no two quarries, or beds of 

 the same quarry, or even distant parts of the same bed, are alike in structure, aspect, 

 hardness, durability, or in the abundance of their included organic relics ; and they appear 

 to have constituted a deposit both littoral and formed in a shallow sea, exposed to the 

 influence of tides and currents. The beds, which are sometimes of considerable thickness, 

 consist of layers of testacea, in a fragmentary state, piled confusedly, but forming, obliquely, 

 laminated surfaces, often interrupted and crossed by others which proceed in different 

 directions. The shelly relics often constitute a considerable proportion of the whole mass ; 

 they are converted into crystalline carbonate of lime, which frequently fills the interior of 

 the univalves ; and it is to the abundance of this mineral, disseminated everywhere, that 

 the weatherstones owe their superior durability upon exposure to the atmosphere. As a 

 general rule, therefore, the beds which contain the greatest abundance of shells are those 

 which are most fitted to resist the action of frost ; water percolates their structure in much 

 smaller quantity, and more slowly, and, on escaping, carries away but little lime in 

 solution. The open joints of the Great Oolite, adjacent to the shelly beds, are therefore 

 nearly free from the large stalactitical masses which load the joints of the freestone in the 

 Inferior Oolite. 1 With the testaceous fragments are associated shells in a perfect condition, 

 though frequently worn and abraded, the valves of the conchifera being rarely in apposition ; 

 also, palatal bones and teeth of fishes, portions of Crustacea, spines of cidaris, ossicula of 

 pentacrinites and asterias, rolled fragments of zoophytes, and dicotyledonous wood, the 

 partitions of the beds disclosing not unfrequently the ripple-marks of a beach. It might 

 be imagined that beds of such a littoral character would be unsuited to the propagation 

 and development of the Cephalopoda ; and it will occasion no surprise when we find that 



1 For economic purposes, the weatherstones are valuable on account of their durability. In proof of 

 this, we may refer to the good state of preservation which the ancient part of the church at Minchinhampton 

 exhibits, and which shows a care and judgment in the selection of materials not always to be found in 

 modern edifices. It is rather a singular fact, that Caen and Minchinhampton, the two places which have 

 produced by far the most extensive series of Great Oolite shells, are connected historically as well as geologically. 

 Matilda, wife of William the Conqueror, founded the nuns of the Holy Trinity at Caen, of which body one 

 of her daughters became a member. William endowed them with the manor of Minchinhampton, at which 

 place they had a religious establishment. They built the church, and dedicated it to the Holy Trinity. 

 It would seem that William despoiled the Saxon Countess Goda of the manor, which she possessed in the 

 time of Edward the Confessor, and bestowed it upon the favorites of his wife. 



