1C ROCKS. 



iries, those which are rudely stratified, laminated, foliated or slaty, as gneiss, mica slate, clay slate ; 

 id those which areunstratified, as quartz rock, and the perfectly crystalline limestones or marbles 



Metamorphism, or the changes the various strata have undergone, may have arisen from the 

 Tects of heat, heated vapours, gaseous exhalations, or the proximity of igneous rocks ; and 

 lese changes may have been different, according to the localities. Thus, in some places, 

 lere may be simply a re-arrangement, or alteration of the mineral substance, as the conver- 

 on of an earthy into a crystalline substance; others may have undergone an entire change, 

 r even loss of a portion of their substance; a third change may have effected the introduction 

 r elimination of minerals in some localities, which are not generally found in others; and a 

 mrth change may entirely alter, or even obliterate the original character, and produce a new 

 :ructure in the rock, as in slaty cleavage. 



Although many of the Metamorphic Rocks are merely the altered palaeozoic strata, and con- 

 jquently referred to the primary series, still there are others of a considerable later date, 

 'or as igneous action has been in operation during every period of the earth's history, so it is 

 robable that different strata have been successively changed. Thus, some of the limestones 

 r finer marbles of the south of Europe, as that of Carrara and other localities, which were 

 >rmerly considered to belong to the primary series, are now ascertained to be of the age of 

 ic Jurassic rocks. The calcareous and argillaceous strata belonging to the lias, in the western 



[lands of Scotland, (Portree, for example) have been converted into highly compact limestones 

 id a species of lydian stone. The basaltic rocks and dykes which form so prominent a feature 

 n the north coast of Ireland, have effected a change in the earthy chalk of that vicinity, (as 

 i the Island of Raghlin), with which they are in contact, converting it into compact, and 

 Dmetimes granular limestone. 



The ordinary roofing slates belonging to the clay-slate group, are the result of metamorphic 

 ption. These argillaceous strata were originally deposited as fine sediment at the bottom 

 f the sea, and have been subsequently elevated from their original position, consolidated and 

 contorted; and have been also subjected to the operation of other 

 forces, which have produced a peculiar structure or slaty cleavage, 

 which cleavage is very uniform over large areas, and generally 

 obliterates the original planes of stratification, and rarely coincides 

 with them. In the accompanying figure, the undulating lines 

 are the planes of bedding, and the oblique lines are those of slaty 

 AGE. cleavage. 



IEYNOLDS, 174, STRAND. 



