CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 3 1 



moderately high temperature and impregnated with water holding 

 more or less powerful chemical agents in solution. 



The initial stages of the changes above alluded to can be well 

 observed in many of the older limestones, where the rock has been 

 subjected to sufficient pressure to produce crushing and cleavage, 

 but where crystallisation has been imperfectly or not at all induced. 

 Some of such limestones show plain signs of intense pressure in the 

 distortion and partial destruction of their contained organic frag- 

 ments, as seen in microscopic sections, at the same time that the 

 mass of the rock has remained free from crystallisation. In other 

 cases, as in some of the Devonian limestones of Devonshire, not 

 only are the organic remains in the rock more or less distorted by 

 pressure, but they have usually undergone recrystallisation, though 

 this has not been sufficiently intense to render them unrecognisable. 

 The complete development of the changes here in question is seen 

 in statuary marble and in the " metamorphic " limestones generally, 

 where a microscopic examination of the rock shows it to be a mere 

 aggregate of variously-sized crystals of carbonate of lime (or, in the 

 case of dolomites, of the double carbonate of lime and magnesia), 

 all traces of organic structure being entirely obliterated (fig. 10). In 

 some instances (as, for example, in the case of the white statuary 

 marble of Carrara) it can be shown that such a purely crystalline 

 limestone was, to begin with, a quite normal limestone, which was in 

 part caught up in the folds of a mountain-chain, and thus subjected 

 locally to enormous pressure. In other cases, we have evidence that 

 a whole region has been subjected to powerful earth-movements, the 

 pressure evolved in which has been so intense and so widely diffused 

 that no part of the original limestone has preserved its primary 

 organic structure. In such cases — as, for example, in the crystal- 

 line limestones of the Highlands — adventitious minerals, such as 

 serpentine, are commonly developed in the rock, showing that active 

 chemical changes have accompanied the mechanical pressure to which 

 the rock has been subjected. We must not, however, lose sight of 

 the possibility that the " metamorphic " limestones of the Archaean 

 period (such as the Laurentian limestones of Canada) may owe their 

 crystalline character and their mineral peculiarities, not to alteration 

 subsequent to deposition, but to the conditions under which they 

 were originally formed. It is also to be remembered that in some 

 cases we meet with beds of granular and crystalline limestone inter- 

 calated in a series of more or less completely normal limestones, 

 without there being any obvious reason for the difference. In such 

 cases, it is probable that the entire series of deposits has been sub- 

 jected to pressure, and that, owing to slight peculiarities in mineral 

 or chemical constitution, certain bands have undergone crystallisa- 

 tion, while others have escaped with nothing more than a certain 



