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NEW YORK STATE MUSEUM 



altered in character. The change consists, for the most part, in 

 a reerystallization of their constituent particles, destroying their 

 original textures and structures and developing new ones. The 

 Grenville and Saranac rocks are the ones most affected and in 

 large part have their original characters utterly destroyed. In 

 the great igneous masses the changes are mot so widespread and 

 profound, so that often there is at least a partial preservation 

 of their original characteristics. 



The old sedimentary rocks have lost all traces of lamination 

 and nearly all signs of original bedding; they have undergone 

 complete reerystallization, entirely obliterating their old textures, 

 and, as a result of severe compression, have had a development 

 of cleavable minerals along certain parallel planes, the mineral 

 particles having a common orientation. This gives rise, on the 

 part of the rock, to a capacity to split along such planes, and the 

 structure is a variety of cleavage, and is known as foliation. 

 Sometimes this new structure is parallel to the old bedding 

 planes and sometimes it is not ; often the latter can not be made 

 out at all. In general the old limestones, now converted into 

 coarse marbles, are the only sedimentary rocks which are not now 

 foliated. This is because of the facility with which such rocks 

 become crystalline, their rather uniform composition, such that 

 they consist mainly of one mineral, and their comparatively great 

 plasticity under pressure. 



The great igneous masses are, in general, much less foliated, 

 though this is not true of many of the smaller ones, and specially 

 not of the older ones, those that seem to be of Grenville age. In 

 considerable part the absence of foliation in much of the igneous 

 rock is thought to be due, as in the limestones, to the fact that the 

 rock is largely constituted of a single mineral. Much of the 

 anorthosite, and to a lesser degree of the syenite also, is quite 

 purely feldspathic, the minerals which are most effective in pro- 

 ducing foliation being present in but slight quantity, or not at all. 

 Such rocks are often badly mashed and granulated, indicative of 

 the great pressures which they have experienced, but with no 

 production of foliation. But, with change in the rock composi- 

 tion, with the formation in quantity of biotite, amphibole or 

 pyroxene, more or less foliation is pretty- sure to be induced in 



