31 



matra (1). There is a source less remote of motion in the upper portion of volcanic andplu- 

 tonic fluid or semi-fluid masses which I have not seen noticed by geologists. When the superin- 

 cumbent strata were fractured in NW. — SE. lines, and the fluid mass pressed into the 

 openings, the adjoining portions woutd acquire a temporary motion towards the openings or 

 at right angles to the lines of fracture. In all cases of plutonic elevation there must either 

 be a slow continued motion upwards as the strata insensibly give way to the pressure , or an 

 accelerated motion when great rents in the strata are abruptly produced. In most cases the 

 viscid granitic mass has probably been of too great extent, and the motion too slow and uni- 

 form ,■ to give rise to any variableness of structure. 



If a motion , howevcr induced , of the igneous fluid from SW. to NE. be assumed , the 

 alternations which we find in the Pulo Ubin rocks , in bands running SW. — NE. , might 

 perhaps be explained on the same principle by which Professor Forbes accounts for the viscid 

 mass constituting a glacier being zoned in the direction of its length , — an explanation which 

 Mr. Darwin has applied to the lamination of volcanic rocks of the trachytic series. In the 

 case of the Pulo Ubin rocks , the general agreement in direction between the zones of 

 variable mineral character and the principal joints , renders it in a high degree proba- 

 ble , if not certain, that, if not contemporaneous in origin , the continued action of the 

 same cause superinduced both. This cause must have begun to operate when the mass 

 was in a fluid or viscous state. Now as some zoned glaciers are of as great bulk as many 

 exposed granitic masses, such as those of Cornwall or that of Pulo Ubin, we may safely 

 consider that the influx of a viscous body of granite into fissures gradually enlarging into 

 great cavities, would, at least sometimes, under a certain range of motion, and where the 

 conditions assimilated to those of glacier motion , be attended with mechanical structural 

 effects somewhat similar to those observed in glaciers, and repeated in the experiments 

 made by Professer Forbes on olher viscid substances. The conditions under which plutonic 

 masses rise must vary very greatly, but there is no dimculty in believing that they some- 

 times, and particularly in masses of no great bulk, approximate sufïiciently to those which, 

 in glaciers , produce parallel bands of variable tension , vertical towards the surface. 



Reverting now to the analogies between the rocks of Pulo Ubin and rocks of a decided 

 gneissose structure (2) , let me request that , in reference to this subject , the preceeding dis- 

 cussion be kept in view. Mr. Darwin remarks that such facts as the vertical or highly in- 

 clined lamination of felspathic rocks , such as he observed at the island of Ascension, and 

 which exist elsewhere , » are manifestly of importance with relat ion to the structural origin 



(1) In the most recent instauce of an earthquake of great power , — that experienced on the west coast of 

 South ufmerica in 1835 and the phenomena of which clearly proved the identity of plutonic and volcanic power , — 

 the undulations appear to bave proceeded from the SW. extending consequently in NW. . . SB. waves. Mr. 

 Dabwu say«: » The fissures in the ground generally, though not uniformly , extended in a SE. and NW. 

 direction and therefore corresponded to the lines of undulalion or of principal flexure." Dabwis's Journal p. 311 

 (2d. ed.) 



(2) JnU p. 20. 



