THE DEVONIAN LIMESTONES OF SOUTH DEVON. 37 



24. On the MiCEOscoPic Strfctuee and Eesidtjes Insoluble in 

 Htdeochloeic Acid in the Devonian Limestones of South 

 Devon. By Edw. Wetheeed, Esq., E.G.S., E.C.S., E.E.M.S. 

 (Read March 23rd, 1892.) 



[Plate IX.] 



Contents. 



Page 



§ 1. Introduction 377 



I 2. Detailed examination of the Organic Structure 377 



,§ 3. Conclusions drawn from exaiuinatio-n of the slides 380 



§4. Residues insoluble in Hydrochloric Acid 382 



§ 5. Detailed examination of the Residues 383 



§ 6. Conclusions drawn from examination of the Residues... 385 



§ 1. Inteoduction. 



The complicated nature of Devonian Geology in South Devon is 

 well known. In this paper I shall confine my remarks to the 

 microscopic examination of the limestones, and shall not enter on 

 the stratigraphical and palgeontological problems which have been 

 worked out by such geologists as De la Beche, Jukes, Godwin- 

 Austen, HoU, Champernowne, Etheridge, Ussher, Davidson, Whid- 

 borne, H. B. Woodward, and others. 



Eor the purpose of collecting specimens of limestone to be exa- 

 mined I spent a mouth in South Devon during the summer of 1890, 

 and I desire to express my thanks to Mr. W. A. E. Dssher, of H.M. 

 Geological Survey, for the very ready assistance and friendly advice 

 which he gave me. 



§ 2. Detailed examination op the Oeganic Steuctuee. 



(a) Hope's Nose Limestones. — At Hope's Nose, near Torquay, there 

 is a quarry opened in limestone of varying light and dark colour, 

 and this is capped by a shaly limestone, which Mr. Dssher regards 

 as belonging to the Caleeolen Kalk, " apparently completely inverted 

 upon massive bedded limestone containing Heliolites porosus.'^ ^ 



Commencing with the Calceola Beds, as representative of the 

 basement limestones, thin sections, seen through a microscope, show 

 a light grey, finely-crystalline groundmass traversed by fissures 

 filled with calcite. In the groundmass are ferruginous patches and 

 minute rhombohedral crystals, apparently of dolomite. 



Passing to the limestones below — which, it must be remembered 

 are assumed to be above the Calceola Beds when in their normal 

 position — thin sections of the lowest beds exposed show them to be 

 composed of broken calcareous fragments, the structure of which 

 has, for the most part, been obliterated. Those which could be deter- 

 mined are fragments of corals, shells, and one or two ossicles of 



1 Quart. Journ. Geol. See. vol. xlvi. (1890) p. 500. 



