80 GEOLOGY OF THE LOTHIANS. 



occur as veins, of an origin contemporaneous with the appa- 

 rently fragmentary rock ; and farther, the pseudo fragments 

 are all found, when the true fracture of the rock is obtained, 

 to pass gradually into the containing basis. 



In the augitic series of the Lothians there are greenstones 

 which, from containing angular masses of greenstone, im- 

 bedded in a trap basis, might in some cases be confounded 

 with a true tufa. In many instances, however, there may 

 be observed a gradual transition of the apparently tuface- 

 ous rock into a perfectly well characterized greenstone ; a 

 fact which at once appears to prove, that the fragmentary 

 aspect may, in some instances, be deceptive. It is almost 

 impossible to conceive circumstances adequate to produce the 

 passage of a true; tufa — a rock which is of an evident mechan- 

 ical origin — into greenstone, which is a true aggregate of 

 more or less perfect crystals, and has resulted from the cool- 

 ing of a fused mass. It may be said, that the long action of 

 the greenstone upon the tufa, might make the semblance of 

 a transition. When trap-tufa, however, from a proximity to 

 such a mass, appears to have been altered, the appearances 

 are only of that description which, in some instances, at- 

 tend the junctions of true aqueous rocks with Plutonic 

 masses. Throughout the whole extent of the Pentland 

 range, there are but few localities where the connexions of 

 the igneous with the stratified rocks are well exposed. Near 

 the extremity of the valley of Glencorse., which runs in a 

 N. E. and S. W. direction through the Pentlands, and be- 

 side the ruins of an old chapel, sandstone occurs, dipping 

 S. at 15° or 20°, thus resting upon the porphyry which 

 forms the north side of the glen, and sinking under that 

 which composes the hills on the south. As to the appear- 

 ance exhibited at the junction of the sandstone with the por- 

 phyry nothing can be said : throughout its whole visible 

 extent, however, it is changed, in some places passing into 



