1 8 INTRODUCTION. ' 



example, with some limestones of quite recent date, and with the so- 

 called " lithographic " limestones of various geological periods from 

 the Ordovician onwards. Similar fine-grained calcareous muds may 

 likewise be formed by the slow digestion and consequent disintegra- 

 tion of the calcareous skeletons of animals and plants, which takes 

 place when such skeletons are long exposed to the action of sea- 

 water. In other cases, again, a calcareous mud of the kind here 

 spoken of may be produced by the wearing down of previously exist- 



Fig. 2. — a, Thin section of lithographic limestone, Jurassic, Solenhofen, greatly magnified. 

 The rock is a fine-grained calcareous mud, for the most part non-crystalline, b, Thin section 

 of an arenaceous limestone from the Middle Permian of Westmorland. The rock is largely 

 mechanical in origin, angular fragments of quartz being cemented together by a crystalline 

 dolomitic matrix. (Original.) 



ing limestones. More commonly the abrasive action of the sea has 

 not been sufficiently prolonged or severe to reduce the calcareous 

 fragments to the form of mere calcareous grains, in which the organic 

 structure is no longer perceptible. Hence the great majority of 

 limestones, when examined microscopically, are found to consist of 

 more or less complete skeletons, or portions of the skeletons, of dif- 

 ferent kinds of lime-producing animals or plants, cemented together 

 by a general crystalline or granular matrix. The general mode of 

 origin of such limestones is rendered sufficiently clear by an investi- 

 gation of calcareous deposits now in process of formation. Such a 

 limestone to begin with exists in the form of an accumulation of 

 entire or fragmentary calcareous skeletons, of all shapes and sizes, 

 loosely heaped together, and more or less extensively separated by 

 vacant spaces. In the process of consolidation, the irregular lacunae 

 between the component fragments of the mass may become infil- 

 trated with fine calcareous mud, produced by the disintegration and 

 wearing down of the superficial portions of the mass ; and the result- 

 ing rock will then have the structure of a granular matrix enclosing 

 innumerable entire or fragmentary organisms. An excellent example 

 of such a rock is to be obtained in the White Chalk (fig. 3), which 

 consists of innumerable organic fragments, mostly referable to the 



