.2 20 The Scottish Naturalist. 



for some long distance westwards from Moncreiffe Hill, and is 

 a filled-up fissure that probably resulted from the Perth volcanic 

 phenomena. The structure of this is cuboidal prismatic — that 

 is, the general appearance is that of a mass of irregular cubes 

 piled up in the wildest possible manner, presenting on a small 

 scale most picturesque cliffs. The cube forms are in no wise 

 attached to each other, the filling-up material being decayed 

 trap. This structure gives us an idea of how easily volcanic 

 masses that are scattered over the country were removed. One 

 of these, that lay at the bottom of a rising-ground, presented to 

 my admiring eyes most beautiful glacial marks, when I was 

 heartlessly informed that it had "humbugged the plough for 

 many years before it was raised." I have noticed that cuboidal 

 masses sometimes become spherical as their coats wear off. 



It is, to say the least, curious that volcanic rocks should assume 

 a great variety of aspects, combined as they are with great sim- 

 plicity of composition, about nine chemical substances forming 

 the whole. The felspathic and hornblend families produce from 

 their union nearly all the other varieties. Hornblend seems to 

 be but augite slowly cooled. Changes are wrought among these 

 rocks as easily as changes upon bells. Even the rate of cooling 

 and pressure are of the first importance. If mica is added to a 

 crystalline felstone, or quartz to a crystalline greenstone, either 

 becomes a granite. From the debris of the Moncreiffe Hill 

 tunnel, I procured crystalline felstone, greenstone, and trap- 

 pean ash or tuff, forming a breccia of angular pieces, embedded 

 in a fine-grained base, and other nameless varieties probably 

 representing all the species to be found between Samson's Ribs 

 on the one hand and Staffa on the other. These traps are 

 ■certainly " roches hors de serie" 



Some of these pyrogenous rocks have cooled quickly near the 

 surface of the earth ; such are generally cellular or scoriaceous, 

 having been easily expanded by the pent-up gases. Others 

 cooled slowly deep in the earth, or on the floor of the ocean, 

 under great pressure. These are more dense and crystalline. 

 Several of the serial strata present joint planes (limestone for 

 instance), often running at right angles to the lines of deposition. 

 In volcanic rocks, in order to form cubes, we must have three 

 sits of joints, one crossing the other two at right angles. In 

 prismatic columnar structures, the joint planes appear to have 

 begun at the top, where first cooled, and to have struck down- 

 wards. A block Avill often break with a smooth surface, or as 



