GIRVANELLA IN OOLITIC EOCKS. 273 



limestone originate from the GirvaneUa-tuhuli there is proof, but 

 in these the structure, though it can be made out, is almost obli- 

 terated. 



Eed No. 2 consists of dark nodular limestone and argillaceous mate- 

 rial. The sectioijs of the tbrmer show it to be extensively crytalline, 

 the calcite beiug slightly tiuged with iron-oxide. Scattered here and 

 there are some Poraminifera ; but the most conspicuous feature is 

 formed by some oolitic spherules which measure as much as 3 millim. 

 in diameter (fig. la). Viewed in a thin section of the rock, these are 

 seen to be masses of tubuli (fig. 1 6), measuring only -01 millim. 

 in diameter, forming a crust around a nucleus. This nucleus is 

 usually calcite, and probably represents some foreign object which 

 served as a nucleus to which the organisms attached themselves, and 

 which has changed into calcite. I X)ropose to distinguish this species 

 by the name of Girvanella incrustans. 



Eed No. 3 is a blue limestone largely made up of Foraminifera, 

 associated with oolitic spherules. In this bed we again meet wdth 

 the organism I propose to call Girvanella Bitcii, occurring in loose 

 aggregations, as represented in fig. 2 a. It is also found surround- 

 ing foreign objects, as in fig. 2 6, in which case an oolitic granule. is 

 made up. In fig. 2 b there may be said to be three nuclei, which 

 serve as centres of growth. With the largest of these the very 

 interesting question is again raised as to whether the concentric 

 arrangement around the nucleus is of organic or inorganic origin. 

 The original structure is, doubtless, much obliterated ; but the 

 outlines of tubes can be seen, and I am therefore disposed to regard 

 it as organic. G, Ducii may be said to be numerous in this bed, 

 sometimes in loose aggregations of tubules, at other times growing- 

 round some foreign object. 



Bed No. 4 consists of an oolitic limestone in which the spherules 

 are unusually dark in colour. In my slides, made from this bed, I 

 obtained one specimen of a loose aggregation of G. Ducii, in which 

 the tests were for the most part well preserved. 



The next development of oolitic limestone occurs just above what 

 is known as the " Great Pault," where the Millstone-Grit is thrown 

 down against the Middle Limestone. These oolites rest on dolomi- 

 tizcd limestone, and are like the others previously referred to, which 

 rest on a like basis. That is to say, the nuclei of the spherules are 

 seldom preserved, and the concentric arrangement is highly crys- 

 talline. The radiate crystalline structure is prominent and at times 

 develops what Dr. Sorby calls the " fan-shaped arrangement of the 

 small prisms of calcite " *. 



The following analysis of beds on which the oolite rests shows 

 them to be dolomitized : — 



* Presidential Addres?s, Quart. Journ. Geol. Soc. [>. 71. plate x. fig. 1 

 (1870). 



