CLASSIFICATION OF LIMESTONES 351 



posing them must be preexisting limestone beds. Their fine stratifica- 

 tion, wliich has procured for many of them the name of ribbon lime- 

 stones, seems to point clearly to normal sedimentation of detrital 

 material as their source. These limestones all contain a considerable 

 amount of clay intimately mixed with the lime, which makes them fit 

 for a natural cement rock. Fossils were well preserved in these muds, 

 and their paucity must be considered as due to the scarcit}^ of animal 

 life in the seas where these deposits were accumulated. 



It will be noticed that in each of the three groups we have limestones 

 which were derived directly from the original organic deposits, or chemic 

 precipitates ; and those which were secondaril}^ derived from older lime- 

 stones, these in turn being generally of clastic origin. The first may be 

 considered as clastic limestones (calcirudites, calcarenites, or calcilutites) 

 of the first generation, the others of the 7i'^ -f 1 generation, in which n 

 may be the first or any later generation. 



Suggested Classification of Limestones 



By the foregoing considerations we are led to divide limestones, as a 

 whole, into two main classes, the non-clastic and the clastic. As Walther 

 has well maintained,*** the metamorphic phase of these limestones should 

 not be considered a primary division. 



I. Under the non-clastic limestones we have : 



A. Chemically deposited limestones, which include: 



(1) Original precipitates in the sea or lake, including chemically 



formed oolites. 



(2) Local precipitates from redissolved limestones. Among these 



are: 



(a) Cave deposits — that is, stalactites and stalagmites. 

 (&) Travertine or calcareous tufa. 



B. Organically formed limestones, which include : 



(3) Unassorted or non-stratified organic limestones, such as reef 



rock, made of corals, bryozoa, hydrocorallines, calcareous 

 algae, etcetera. 



(4) Assorted or stratified organic limestones, such as — 



(c) Shell beds, composed of shells of mollusks, brachiopods, 



echinoderms, and the larger foraminifera— that is, 

 Fusulina, Nummulites, etcetera. These grade imper- 

 ceptibly into the clastic shell conglomerates and 

 breccias. 



(d) Shell ooze limestones, including chalk, globigerina 

 \ ooze, pteropod ooze, entomostraca ooze, etcetera, and 



the limestones resulting from their consolidation. 



* Johannes Walther: Versueh einer Classification der Gesteine auf Grund der vergleiehenden 

 Lithogenie. Congr^s Geologique International Compte rendu de la Vllme. Session. St. Peters- 

 Dourg, 1897, 3m e Partie, pp. 9-25. 



