222 PREVAILING THEORY CONCERNING [Ch. XT. 



varying greatly in composition, colour, and thickness. For 

 instance, gneiss alternates with layers of black hornblende- 

 schist, or with granular quartz or limestone, and the inter- 

 change of these different strata may be repeated for an 

 indefinite number of times. In like manner, mica-schist 

 alternates with chlorite-schist and with granular limestone, in 

 thin layers. As we observe, in the secondary and tertiary 

 formations, strata of pure siliceous sand, alternating with 

 micaceous sand and with layers of clay, so in the primary we 

 have beds of pure quartz-rock alternating with mica-schist 

 and clay- slate. As also, in the fossiliferous series, we meet 

 with limestone alternating again and again with micaceous or 

 argillaceous sand, so we find the gneiss and mica-schist alter- 

 nating with pure and impure granular limestones. Reasoning, 

 therefore, from the principle that like effects have like causes, 

 the stratification of these primary rocks may be attributed to 

 sedimentary deposition from a fluid. 



But, if gneiss is a sedimentary rock, how comes it to pas& 

 that it frequently passes into granite, which is an igneous 

 rock ? The Huttonian hypothesis offers the following solu- 

 tion of this problem. The materials, of which the gneiss i& 

 composed, were originally deposited from water in the usual 

 form of aqueous strata, but these strata were subsequently 

 altered by their proximity to granite, and to other Plutonic 

 masses in a state of fusion, until they assumed a granitiform 

 texture. 



The geologist has been conducted step by step to this 

 theory, by direct experiments on the fusion of rocks in the 

 laboratory, and by observation of the changes in the com- 

 position and texture of stratified masses, as they approach or 

 come in contact with igneous veins or dikes. Thus, the shale 

 and limestone of Anglesea, when it approaches a dike of 

 basalt at Plas Newydd is, according to Professor Henslow, 

 gradually changed into porcelainous jasper and crystalline 

 limestone. In the hardest part of the mass, the fossil shells 

 are nearly obliterated : but the most extraordinary circum- 



