their geographical distribution, and that they must not therefore be regarded 

 as fixing absolutely the age of the rock when found in any new locality. 



Consider, now, the plutonic rocks. These are much more difficult to 

 deal with because, as they always occur in the form of intrusive masses, it 

 is in many cases impossible to determine their age with precision. All that 

 can be said with confidence is that they are later in date than the rocks into 

 which they have been intruded : how much later we cannot in many cases 

 determine. They are more abundant amongst the older than amongst the 

 younger rocks ; but, as already pointed out, it is not safe to infer from this 

 that they were more frequently produced during the earlier, than during the 

 later geological periods. In any case, they are not surface products, and 

 therefore the differences which they present when compared with volcanic 

 rocks (e.g., granular texture, abundance of liquid and rarity of glass 

 inclusions) cannot be taken as proving differences in the conditions under 

 which the older and younger igneous rocks have been produced. These 

 differences cannot be correlated, as FRENZEL supposes, with phases in the 

 developmental history of the planet. 



Two of the best authenticated cases of plutonic rocks of Tertiary age are 

 the peridotites and gabbros of the Western Isles of Scotland/ 1 ' and the 

 granular diorite of Mount Davidson. (2) These present no differences, so far 

 as composition, texture and inclusions are concerned, from the corresponding 

 rocks of pre-tertiary age. In all probability future researches will greatly 

 increase the number of plutonic rocks of pre-tertiary age. We have been 

 reasoning in a circle to a very great extent with reference to this question. 

 Plutonic rocks have been usually placed as pre-tertiary without the 

 determination of their age by independent field evidence. It is, of course, 

 possible to make a classification which shall be consistent with itself by 

 adopting this method ; but such a classification may not be consistent with 

 the facts of nature. 



If, now, we turn our attention to the secondary features of igneous rocks 

 it is possible to establish a much closer relation between age and 

 petrographical characters. The older rocks have been much more altered by 

 weathering and by regional metamorphism. Pre-tertiary basalts and 

 dolerites have been frequently, but not always, converted into diabases and 

 melaphyres, by the former ; and less frequently into chloritic and horn- 

 blendic schists by the latter. Rocks containing glassy matter have been 

 devitrified, and hyaline rhyolites have thus been changed to felsites. 

 Peridotitic, hornblendic, and augitic rocks have been serpentinised. At the 

 same time the relation is not absolute. Here and there the older rocks have 

 been preserved and the modern rocks affected. It is unnecessary to enter 

 upon a fuller discussion of the secondary characters of rocks at present. 



(1) ZIRKEL, F. Geologische Skizzen von der Westkuste Schottlands. Z.D.G.G., 1871, p. 1-124. 

 JUDD. On the Secondary Rocks of Scotland. Q.J.G.S., vol. XXX., 1874, p. 220. 



JTTDD. On the Tertiary and Older Peridotites of Scotland. Q.J.G.S., vol. XLL, p. 354. 



(2) HAGUE and IDDINGS. On the development of Crystallisation in the Igneous Rocks of the 

 Washoe District. Bull, U.S. Geol. Survey, 1885. 



