20 



INTRODUCTION. 



are essentially organic in their origin, and are largely made 

 up of the remains of marine animals. Magnesian limestones 

 are, however, very often more or less highly crystalline, and 

 they are very often singularly affected by " concretionary " 

 action, so that their primitive composition and structure is 

 often more or less completely destroyed. 

 " Nor is it only through the agency of animals that lime- 

 stones are built up. Many of the calcareous Alga3 the 

 "Corallines" and " Nullipores " are capable of forming 

 accumulations of lime, sometimes upon a most extensive 

 scale. One of the best examples of a limestone formed 

 principally of the calcareous skeletons of these singular 

 plants is afforded by the so-called " Leitha-Kalk " of the 

 Tertiary series. This limestone is largely composed of 

 nodulated masses, which exhibit no definite structure to the 

 eye, and which were originally set down as " concretions " (fig. 

 6). Microscopic examination of these apparently inorganic 

 masses shows, however, that we have to deal here with the 



Fig. Q.Lithothamniiim ramosissimum, a calcareous Alga, from the Leitha-Kalk of the 

 Vienna Basin, a, Portion of a mass, of the natural size ; b and c, Transverse and vertical 

 sections of the same magnified 320 diameters. After Gttmbel. 



calcareous skeletons of a kind of Nullipore (Litliothamniuni). 

 The Leitha-Kalk is not only extensively developed in the 

 Austro-Hungarian empire, but can be traced through Bosnia 

 into Turkey, and appears to be continued through Asia Minor 

 into Armenia and Persia. Similar calcareous Algae are found 

 in many of the Secondary limestones ; and the Palaeozoic lime- 

 stones will also, doubtless, be found in time to contain the 

 skeletons of these plants to some extent. 



', Phosphate of lime is another lime-salt, which is of interest 

 to the palaeontologist. It does not occur largely in the stra- 

 tified series, but it is found in considerable beds l in the 



1 Apart from the occurrence of phosphate of lime in actual beds in the strati- 

 fied rocks, as in the Laurentian and Silurian series, this salt may also occur 



