chap. vii. Sandstone Platforms. 149 



crystalline metamorphic rocks. I mention this, because 

 I found near Rio de Janeiro a well-defined angular 

 fragment, seven yards long by two yards in breadth, of 

 gneiss containing garnets and mica in layers, enclosed 

 in the ordinary, stratified, porphyritic gneiss of the 

 country. The laminee of the fragment and of the sur- 

 rounding matrix ran in exactly the same direction, but 

 they dipped at different angles. I do not wish to- 

 affirm that this singular fragment (a solitary case, as 

 far as I know) was originally deposited in a layer, like 

 the shale in the Blue Mountains, between the strata of 

 the porphyritic gneiss, before they were metamorphosed ; 

 but there is sufficient analogy between the two cases to 

 render such an explanation possible. 



Stratification of the escarpment. — The strata of 

 the Blue Mountains appear to the eye horizontal ; but 

 they probably have a similar inclination with the sur- 

 face of the platform, which slopes from the west to- 

 wards the escarpment over the Nepean, at an angle of 

 one degree, or of one hundred feet in a mile. 1 The 

 strata of the escarpment dip almost conformably with 

 its steeply inclined face, and with so much regularity, 

 that they appear as if thrown into their present position ; 

 but on a more careful examination, they are seen to 

 thicken and to thin out, and in the upper part to be 

 succeeded and almost capped by horizontal beds. These 

 appearances render it probable, that we here see an 

 original escarpment, not formed by the sea having 

 eaten back into the strata, but by the strata having 

 originally extended only thus far. Those who have 

 been in the habit of examining accurate charts of sea- 

 coasts, where sediment is accumulating, will be aware, 

 that the surfaces of the banks thus formed, generally 



1 This is stated on the authority of Sir T. Mitchell, in his 

 • Travels,' vol. ii. p. 357. 



