GIRVANELLA IN OOLITIC KOCKS. 271 



formation which I think will justify me in drawing a distinction 

 between the Coralline-Oolite form and G. problematica, to which I 

 temporarily referred it. 



II. The Caeboniferous Oolitic Limestones. 



In the Carboniferous Limestone of Gloucestershire oolitic lime- 

 stone is more common than is generally supposed. Taking the gorge 

 of the Avon as the typical section, I find oolitic limestone to occur 

 at four horizons in the middle series. It is also worthy of remark 

 that in three instances out of the four the oolitic limestones rest on 

 doloraitized limestone. 



1. The first development of oolite occurs at the base of the 

 Middle Limestones, and the following analysis shows the beds to be 

 dolomitized : — 



Insoluble residue 1*40 



Organic matter 6-20 



Magnesia 15"56 



Lime 32-30 



Carbonic acid 42*78 



Oxides of iron and alumina 1-10 



Alkalies, &c -60 



100-00 



2. About 100 feet of oolite follow above the dolomitized limestone, 

 consisting of well-defined spherules of dark colour and granular 

 crystalline structure. These stand out in relief against the clear 

 calcite which occupies the space between the spherules, and average 

 about -6 millim., but occur as small as -1 millim. in diameter. In 

 some the main axis has been extended and a spheroidal form has 

 been developed, which is usually larger than the others. In some 

 spherules the crystalline radiate feature is seen, but the concentric 

 arrangement can always be distinguished. In most instances the 

 original nature of the nucleus cannot be determined, it having passed 

 into calcite. AVhen a nucleus is preserved in its original form 

 it is usually a Foraminifer in the spheres, and in the spheroids a 

 spicular object which could not be definitely distinguished, pro- 

 bably a portion of a small spine. 



3. The next development of oolitic limestone is in what is known 

 as the Great Quarry, and, like that previously mentioned, it rests on 

 dolomitized limestone, as the following analysis of the beds shows : — 



Insoluble residue 2-70 



Organic matter 5-20 



Magnesia 11 -SO 



Lime 42-48 



Carbonic acid 37*40 



Oxides of iron and alumina trace. 



Alkalies, &c -30 



100-00 



