Vol. 56.] MECHANICALLY-FORMED LIMESTONES FROM KATHIAWAR. 559 



29. Mechanically-formed Limestones from Junagarh(Kathiawar), 

 and other Localities. By John William Evans, D.Sc, LL.B., 

 F.G.S., formerly State Geologist to the Junagarh State. (Read 

 June 6th, 1900.) 



Contents. 



I. The Original Structures of Mechanically-formed Calcareous Rocks 



— Subaqueous, Littoral, and iEolian 559 



II. The Junagarh Limestone 561 



III. Other Kathiawar Rocks of Similar Character 565 



IV. Deposits in Kach , 567 



V. Similar Beds on the South-eastern Coast of Arabia and the Islands 



in the Persian Grulf 567 



VI. The Conditions under which the Deposits in the Neighbourhood 



of the Arabian Sea were formed 569 



VII. Other Tropical and Subtropical Deposits of like Nature 574 



VIII. Colder Regions : the Foraminiferal Deposits of Dog's Bay (Galway) 576 



IX. The Oolites of the Jurassic Period 578 



X. Bibliography 581 



I. The Original Structures of Mechanically-formed Calcareous 

 Rocks — Subaqueous, Littoral, and ^Eolian. 



We are apt to think of the calcareous rocks as being laid down 

 differently from other sedimentary strata. It is recognized at once 

 that the deposition of the latter involves the mechanical forces of 

 ^rivers, currents, waves, or winds ; but the former — when they are 

 not the result of chemical precipitation — are often spoken of as if 

 they represented merely the undisturbed accumulation of calcareous 

 material, which either remains where the organism, of which it 

 formed part, ceased to live, or sinks by its own weight at once to 

 "the sea-bottom. 



This distinction is, however, as every one would admit, only very 

 partially valid. All the structures characteristic of the sedimenta- 

 tion of arenaceous beds are exhibited, on occasion, by calcareous 

 rocks ; though when these are formed in the sea, the phenomena are 

 restricted and modified by the fact that carbonate of lime is soluble 

 to a very large extent in water containing carbonic anhydride. 1 



As a general rule, the water of rivers, lakes, and seas is not 

 completely saturated with carbonate of lime, and therefore acts as 

 a solvent of that substance. A small calcareous particle — whether 

 it is derived from the erosion of a limestone ; or is the result of the 

 comminution of shells, corals, and other organic remains ; or repre- 

 sents the test of a minute organism — will not remain long suspended 

 in water which is still capable of taking up carbonate of lime, before 

 it disappears into solution. Fine-grained calcareous material will 

 consequently, under ordinary circumstances, be deposited only when 

 there is so little current that the particles speedily sink to the 

 bottom, where the water is saturated with lime from the calcareous 



1 Solution and recrystallization subsequent to the formation of the rock also 

 tend to obliterate these structures. 



2 P 2 



