116 L. W. Fulcher — On the Hirnant Limedone, 



At present I have not the material for this purpose, but I have^ 

 thought it well to mention the fact of the peculiar aspect of these 

 sandstone fragments, as Ordovician plants are so rare that any in- 

 dication of their existence is naturally a matter of interest. I may 

 add that a section which I had prepared of the fragment did not 

 reveal any noteworthy characters under the microscope. 



When a section of the rock is examined under the microscope, w© 

 see the grains lying in a crystalline matrix of calcite (PL IV. Figs. 

 1-2, for the drawings of which I am indebted, to the kindness of 

 Mr. E. W. Wetherell). The centre of the grains consists usually of 

 granular calcite, and in some cases has the form of a figure of 8, but 

 is generally rather irregular in shape. Occasionally the nucleus is 

 the fragment of an organism or a grain of sand. Around the centre 

 the carbon appears as a series of opaque concentric rings, though 

 now and then patches are seen to have been torn away in the pre- 

 paration of the section. In a few cases the carbon seems to fill np the 

 ■whole of the interior of the grain, but this is probably due to the fact 

 that the centre of such grains has not been cut through. Externally 

 the grains are seen to be increased in the direction of their longer 

 axes by a secondary development of a fibrous-looking mineral. This 

 statement, however, must be modified. In all the sections which 

 I had prepared from my own specimens this secondary mineral 

 appears; but Prof. Cole has lately lent me a slide in which it is 

 entirely absent, so that the alteration is possibly only local. The 

 fibrous-looking mineral from its optical properties seems to be a 

 chalcedonic variety of silica. Under a high power the fibrous 

 appearance is seen to be due to a number of thin elongated prisms 

 which are apparently quartz. This mineral is itself bounded by 

 a band of almost opaque material which sometimes includes carbon. 

 There are often three or four bands of chalcedony separated from 

 each other by a layer of the dusty material. 



There is very rarely a radial grouping in the grains as in some 

 Oolites, nor does there ajipear to be any such structure as the recently 

 described Girvanella.^ The grains give no black cross between 

 crossed Nicols. 



The matrix in which the grains are embedded is seen to be 

 ordinary crystalline limestone, but it contains here and there frag- 

 ments of organisms (Polyzoa), and also small irregular grains of 

 quartz, which are by no means regularly dispersed, being in some 

 places much more numerous than in others. These quartz grains often 

 contain moving bubbles. It is also to be noticed that occasionally 

 an extremely thin band of secondary calcite traverses the whole 

 section, penetrating right through the grains. A section which has 

 been mounted in Canada balsam, and then etched with hydrochloric 

 acid so as to dissolve the calcite before being covered up, shows more 

 clearly the concentric structure of the grains and the outer coating 

 of chalcedonic silica which they possess. 



It will be observed from the above description that the rock is 

 somewhat analogous in structure to the Cleveland iron ore,- the 



1 Wethered, Geol. Mag. 1889, p. 106. 



2 Sorby, Q.J.G.S. vol. xxxy. (1879) Proc. p. S4. 



