C ALTON HILL. 01 



at 20° and sink completely below the trap, near the new 

 High School. Immediately below the old Observatory, 

 and on the road which leads round Dugald Stewart's mo- 

 nument, strata of trap-tufa are again found cropping out, be- 

 ing included between the porphyry which overlies the masses 

 of trap-tufa we have mentioned, and underlying those 

 Neptunian deposits which form the western acclivity of 

 the hill. The tufaceous strata may be traced more or 

 less interruptedly till they disappear under the porphyry 

 of Nelson's monument. The changes which the masses 

 of trap-tufa exhibit when in contact with the porphyry 

 are very great, inasmuch as they are intensely indurated. 

 The porphyry also acquires, when next the tufa, an 

 aspect different from that which it exhibits when at a 

 distance from it ; it becomes very compact, and frequent- 

 ly passes into compact felspar. On examining the points 

 of contact of these two rocks, there is frequently ex- 

 hibited an almost gradual transition of the one into the 

 other, so that it requires considerable attention in some in- 

 stances to discover where one rock terminates and where 

 the other begins. All these appearances may, however, be 

 easily explained, if we remember that as both rocks were 

 in all probability once in a fluid and highly heated state, 

 there may have been in some places a complete intermix- 

 ture of the one with the other. Several simple minerals 

 occur in the Calton Hill ; jasper traverses the porphyry 

 in minute veins ; sulphate of barytes occurs in the same 

 manner, and also masses of native coppery* In the porphy- 

 ry immediately below the old Observatory, veins of calca- 

 reous spar are found ; and associated with the calcareous 

 spar, glance-coal is to be met with, forming imbedded masses 

 of various sizes. The situation of this mineral is highly in- 

 teresting, and when the vegetable origin of coal is admitted, 

 its mineralogical characters and relations to the associated 



