140 Scientific Proceedings, Royal Dublin Society. 



becoming more basic, and thus reducing its temperature whilst it 

 became more fluid. If this were the case we can understand that 

 further action on limestone would be limited by saturation of the 

 magma with lime and its low temperature. It therefore seems 

 that we should look more to granite and its derivative as fuses of 

 limestones than to basic rocks. Why should not the basalts of 

 Mull be the result of the contact of the granites with the under- 

 lying limestones? I have brought the subject forward, not with 

 the intention of offering new evidence, but to again direct attention 

 to such an important branch of vulcanological science. 



Let us now turn our attention to the mineral composition of an 

 igneous rock. Any given magma will produce rocks of the most 

 varied character, according to the conditions under which consoli- 

 dation takes place. Thus, for instance, a given dyke of magma 

 might be a granite near its origin, higher up its sides or whole may 

 be pitchstone, and its centre a liparite or porphyry, whilst at the 

 surface it would present itself as vitreous pumice and ash, an 

 obsidian or a quartz trachyte. It has been observed that granite 

 veins branch out, and the ramifications may assume the type of 

 felsite, which is of course dependent on the more rapid cooling, just 

 as in the case of the salband. Again, we have a series of grada- 

 tions from a true leucitic basalt, such as the recent lavas of Vesuvius 

 to a sanidine porphyry, to a highly crystalline syenite containing 

 leucite, but more commonly nepheline in the rocks, composing the 

 ejected blocks. The generalization based upon the geological ages 

 being characterized by different types of rocks is false, and is no 

 doubt due to the depth to which denudation has extended. It is a 

 general fact that the slower the cooling takes place the more perfect 

 will the cystallization be. This we have already spoken of when 

 treating of the difference in the amount of gaseous constituents in 

 a magma which, by bringing about great rapidity of cooling in 

 explosive eruptions, makes the products tend to an amorphous 

 rather than a crystalline condition. One remarkable fact well borne 

 out by the lavas derived from the different eruptions of Vesuvius 

 is that the size of the crj 7 stals are much greater in the little oozing 

 forth of a small quantity of lava from the crater than in the great 

 outpoors. This will be evident, as in the first case the lava has 

 been in a state of simmering in the upper part of the chimney for 

 a long time, and will have been loosing its heat in a very gradual 



