20 



INTRODUCTION. 



organic fragments which compose the mass of the rock. In some cases 

 the matrix has been deposited in the first instance in a crystalline form, 

 and is the result of the percolation through the mass of water charged 

 with carbonate of lime in solution. In other instances, the matrix has, 

 to begin with, been composed of a fine calcareous mud, which has later 

 undergone crystallisation, as the result of secondary chemical and me- 

 chanical changes. Sometimes this superinduced crystallisation may be 

 the result of pressure ; in other cases it may be caused by the per- 

 colation through the rock of heated or carbonated water ; while in many 

 instances it is connected with the process of dolomitisation. 



In this process of superinduced crystallisation, the organic fragments 

 contained in the rock usually show themselves more stable than does the 

 matrix. Hence the matrix may become more or less highly crystalline, 

 while the included organic fragments remain more or less distinct and 

 unaffected. It often happens, however, as specially insisted upon by Dr 

 Sorby, that the fragments of calcareous organisms have crystallised along 

 with the surrounding matrix, in such a manner as to have more or less 

 extensively lost not only their organic structure but also their external 

 outline. There are various degrees in which this superinduced crystal- 

 lisation, and consequent obliteration, of the included organic fragments 

 in a limestone takes place. It is noticeable, however, that the agencies 

 which give rise to this condition are not necessarily of any great intensity, 

 since complete crystallisation and obliteration of the included organic 

 remains may occur in modern calcareous deposits {e.g., some coral-lime- 



Fig. 5. — a, Section of Ordovician limestone, from Keisley, Westmorland, in which the crystal- 

 lisation of the matrix has extended to the included fragments of Crinoids, as shown by the con- 

 tinuation of the cleavage-planes from one to the other. Enlarged about five times. B, Fibro- 

 crystalline structure developed in the same limestone, considerably enlarged. (Original.) 



stones). An interesting example of this phenomenon is commonly seen 

 in Crinoidal limestones, in which the entire rock may be so crystallised 

 that the cleavage-planes of the calcite run continuously through both the 

 matrix and the included fragments of Crinoids, the latter nevertheless 

 preserving their outlines (Fig. 5, A.) 



In many of the older limestones, portions of the rock often exhibit a 

 peculiar fibro-crystalline structure, being composed of feather-like col- 

 umns of crystalline carbonate of lime, placed side by side and intersected 

 by a double cleavage. Sections of such fibro-crystalline masses (Fig. 

 5, B), examined microscopically, commonly show, therefore, a character- 

 istic pinnate or "herring-bone" structure, due to the crossing of the two 

 sets of cleavage-planes. This remarkable structure is obviously the 



