
sedimentary strata near the line of 
racter of marble, each little granule 
Part VIII.§ 2.) LOCAL METAMORPHISM. 577 
Marmarosis.'—The conversion of ordinary dull granular lime- 
stone into crystalline or saccaroid marble may not infrequently be 
observed on a small scale where an intrusive sheet or dyke has 
invaded the rock. One of the earliest described examples of this 
change is that at Rathlin Island off the north coast of Ireland 
(Fig. 298). Two basalt dykes (20 and 35 feet thick respectively) 

bacaca b 
Fic. 298.—Dyxrs or BASALT (@ @ @) TRAVERSING CHALK (0), WHICH NEAR THE DyKES 
IS CONVERTED INTO MARBLE (c), RATHLIN IsLAND, ANTRIM, 
ascend there through chalk, of which a band twenty feet thick 
separates them. Down the middle of this central chalk band runs a 
tortuous dyke one foot thick. The chalk between the dykes and for 
some distance on either side has been altered into a finely granular 
marble. Another smaller but interesting illustration of the same 
change occurs at Camps Quarry near Edinburgh. The dull grey 
Burdie House limestone (Lower Carboniferous), full of valves of 
Leperditia and plants, has there been invaded by a basaltic dyke, — 
which, sending slender veins into the limestone, has enclosed portions 
of it. The limestone is found to have 
acquired the granular crystalline cha- 
of calcite having its own orientation 
of cleavage planes (Fig. 299). 
Production of new minerals. 
One of the results of the intrusion of 
eruptive rock has been the develop- 
ment of crystalline minerals in ordinary 
contact. ‘The new minerals have 
usually an obvious affinity in com- 
position with the original rock. But 
undoubtedly silica has often been Fic. 299.—Sxcrion or LimxstTonx (a) 
introduced as part of the alteration, Ca ea. 
either free or as silicates. MAGNIFIED 20 DIAMETERS. 

An interesting instance of the change was described many years ago 
by Henslow, near Plas Newydd, Anglesea. <A basalt dyke 154 feet in 
breadth there traverses strata of shale and argillaceous limestone, which 
are altered to a distance of 35 feet from the intrusive rocks, the limestone 
becoming granular and crystalline, and the shale being hardened, here 
1 The coining of a new word to express a change for which there is as yet no short 
term may perhaps be pardoned. 
2 Conybeare, Trans. Geol. Soc. iii. p. 210 & Plate x. 
2P 
