GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 4G5 



latter requires only that there be a valley, its direction of 

 dip being immaterial. In either case the tilt of the upraised 

 block necessary for the reversal of drainage would result 

 from the sheet of magma becoming only some 50 or 60 feet 

 thicker at the south end than in the north central part, 

 where the creek leaves the mass ; and this thickening of 

 the sheet might possibly be accounted for by the feeding 

 fissure being somewhat wider, and thus providing a faster 

 flow of magma, at the south than the north. 



It may be noted that acceptance of the hypothesis we 

 venture to offer carried to its logical conclusion would 

 account for the formation of the cavity of the intrusion by 

 the folding of the strata around its edge, if we assume that 

 the valleys were somewhat more steep-sided at that time 

 than now, owing to the then probably greater elevation of 

 the land surface above sea level. It may be granted, we 

 think, that the magma, when it began to spread out, can 

 have had only sufficient upward pressure to produce a small 

 elastic deformation of the shales, whilst lifting them. It 

 overcame their cohesion, and fractured or bent them, only 

 in detail, as it broke across the bedding ; but had not 

 sufficient force to break or bend the whole thickness of 

 superincumbent strata. The shales would be in a condition 

 of elastic strain only over a broad marginal zone of the 

 advancing sheet, and not over its whole surface, the strain 

 being at each spot relieved successively by the bending of 

 shale further on. Thus the sheet would be thin and of 

 uniform thickness over most of its area, but wedge-shaped 

 around the edge, as in the case of any ordinary sill. On 

 penetrating outwards and passing beyond the divides, the 

 magma would cease to find its horizontal progress along 

 the bedding planes checked by an increasing weight to be 

 lifted, and it would now begin to spread out horizontally. 

 As the surface of the ground was now sloping down 



