The Scottish Naturalist. 99 



was forced into them, perhaps many ages before. They are, therefore, 

 called M intrusive," or " subsequent " volcanic rocks. 



The different modes in which the two classes of rocks have con- 

 solidated, give rise to important differences, both in minute structure) 

 and in their occurrence as rock-masses. Thus, interbedded volcanic 

 rocks are always more or less open in texture, particularly in the upper 

 part of the sheets, which generally presents a slaggy appearance, pre- 

 cisely like that of the surface of a recent lava flow. The amyg- 

 daloidal structure is due to the expansion of the rock, while still 

 plastic, by the contained steam. In the case of an intrusive rock, the 

 pressure of the superincumbent mass was too great to admit of such 

 expansion. The steam holes or "amygdules " generally became filled 

 up with various mineral deposits, such as calcite, chalcedony, &c. 

 The amygdaloidal structure is well marked in the interbedded volcanic 

 rocks of Kinnoull Hill ; and where the cavities had been filled with 

 quartz they give rise to the familiar Kinnoull Hill pebbles or agates. 

 The concentric lines, which add so much to the beauty of these, mark 

 the successive layers of the deposit, and may therefore be called 

 " lines of growth." 



There are also certain structural peculiarities which mark intrusive 

 rocks. They are always more fine-grained in their outer than in their 

 central portions, in consequence of the former having been the first to 

 cool and crystallise. They are frequently more or less prismatic. 

 Both of these characters are well seen on examining the intrusive rocks 

 of Kinnoull Hill. 



One of the most important distinguishing features, however, is that 

 while interbedded rocks always conform with the bedding of the rocks 

 with which they are associated, intrusive rocks never do at least in 

 perfect degree. Then again, while the rocks in contact with an in- 

 trusive mass are frequently baked and hardened, and otherwise altered 

 by the molten mass which had been forced against them, that is never 

 the case with rocks containing interbedded sheets. 



How are these two kinds of volcanic rocks represented in the 

 structure of Kinnoull Hill? The great mass of the hill consists of beds 

 of the interbedded or contemporaneous type, while at the back of the 

 hill, cutting through the other rocks, is an extensive vertical dyke of 

 intrusive, or subsequent, igneous rock. The interbedded sheets con- 

 sist of extensive beds of porphyrite, which slope away towards the 

 north-west. It therefore follows that the well-known face or cliff of 

 Kinnoull Hill represents the broken edge, or " outcrop " as it is called* 

 of these beds, which at one time must have extended across the whole 

 of what is now the lower valley of the Tay, but which, in the course 

 of many ages, have been worn away by the denuding action of 

 weather and water. It is thus clear that the corresponding rocks on 

 the opposite side of the valley, which form the main part of Mon- 



