30 INTRODUCTION. 



shape to pressure, but it does not appear that this is an adequate 

 explanation, and their mode of origin is still obscure. 



(4.) Superinduced Structure. — There are probably no limestones, 

 including even those now actually in process of formation, which are 

 absolutely free from superinduced structural peculiarities of one kind 

 or another. In a general way, these superinduced peculiarities 

 depend upon a more or less extensive recrystallisation of portions 

 of the rock, it being sometimes the matrix of the limestone which is 

 thus affected, sometimes the included fragments, and sometimes 

 both. In many cases, the secondary crystallisation of a limestone 

 may be the result of slow chemical or physical changes, connected 

 in the main with the percolation through the rock of water holding 

 carbonic acid or other ingredients in solution. As regards the 

 organic fragments present in most limestones, these gradual changes 

 are doubtless much facilitated by the readiness with which aragonite 

 passes into the condition of calcite. As a general rule, however, 

 these slow alterations do not affect the structure of the limestone so 

 profoundly but that the original constitution of the rock is easily 

 recognisable by suitable methods of examination. In many cases, 

 on the other hand, and especially among the older limestones of the 

 earth's crust, the rock has undergone changes of a much deeper and 

 more far-reaching character than those above alluded to. The most 

 prominent of these changes consists in a more or less complete 

 crystallisation of the rock, leading to a more or less complete oblit- 

 eration of any fossils which it may have contained. The general 

 causes which contribute to bring about this thorough crystallisation 

 of limestones are heat and pressure, singly or together, combined 

 with the action of percolating water, which is rendered chemically 

 potent by having certain substances dissolved in it. 



That the application of a powerful heat to limestone will cause its 

 crystallisation is sufficiently exemplified by the well-known pheno- 

 mena observable in a limestone when intersected by an intrusive 

 igneous rock. Thus, limestone in the immediate neighbourhood of 

 a trap-dyke or a mass of granite is found to have been converted 

 into a crystalline marble, in which, as a rule, no traces of organic 

 structure can be detected under the microscope. When developed 

 upon a larger scale, crystalline limestones are usually found in regions 

 which can be shown to have been subjected to powerful earth-move- 

 ments, one result of which must have been the application to the 

 rocks of the region of intense pressure. Usually greater or less 

 elevation of temperature has co-operated with the pressure in pro- 

 ducing alterations in the structure of the rocks affected by these 

 movements. Speaking generally, therefore, we may regard the 

 " regional " crystallisation of limestones as due to the application of 

 great pressure to deeply buried masses of these rocks, raised to a 



