258 



whenever the term andesite is used in the present work it may be taken 

 to imply a porphyritic texture unless the contrary is stated. Another 

 feature equally characteristic of the andesites is the presence of interstitial 

 matter, either cryptocrystalline, microfelsitic or vitreous. The interior of 

 massive lava flows may become holocrystalline. Then, of course, we have 

 rocks undistinguishable from the hypogene representatives of the same 

 magma (diorite) and the artificial character of our system of classification 

 is exposed. 



The analyses of the diorites of Klausen already quoted may be regarded 

 as representing in a general way the composition of the andesites. 



Trachy-dolerite. This name was introduced by ABICH ( X) for volcanic 

 rocks of intermediate composition. The silica-percentages of the rocks 

 analysed by ABICH range from 57 to 62. The term andesite has now 

 replaced the term trachy-dolerite. Mr. WARD proposed the term felsi- 

 dolerite for certain rocks of the Lake District which resemble the trachy- 

 dolerites of ABICH in composition. These rocks are simply altered andesites 

 and may therefore be appropriately termed porphyrites. 



Porphyrite. A study of the literature does not enable one to attach 

 any very definite signification to this term. It has been applied to rocks, 

 of intermediate and basic composition and even to rocks in which orthoclase 

 is the dominant felspar. The name porphyrite seems to imply a porphyritic 

 texture ; but it has been extended so as to include non-porphyritic rocks. 

 If we define porphyrite as a pre-Tertiary andesite (usually more or less 

 altered) we shall include most of the rocks to which the term has been 

 applied by Continental authors. If we decline to recognize geological age 

 as a factor in petrographical classification we can give some precision to 

 the term by defining it as a more or less altered andesite. A difficulty 

 arises, however, when we endeavour to apply this definition because the 

 term has occasionally been extended to basic rocks. A partial solution of 

 the difficulty would be found if petrographers would agree to call the basic 

 rocks melaphyres (altered rocks of basaltic composition). It is not possible 

 to remove all difficulty because of the existence of intermediate forms. 



In what follows we shall use the term porphyrite for the more or less 

 altered andesites. In this sense typical porphyrites occur in the Lake 

 District and in the Cheviot District. In the latter they are associated 

 with unaltered andesites. COTTA places the porphyrites with the plutonic 

 rocks. The typical porphyrites are unquestionably lavas. They are 

 frequently amygdaloidal and are often associated with tuffs. 



The subdivisions of the porphyrite-group correspond exactly with 

 those of the andesites. We recognize enstatite-, augite-, hornblende- and 

 biotite-porphyrites. The pyroxene-porphyrites (porphyrites with enstatite 

 or augite) are the most basic members of the series and the biotite- 

 porphyrites the most acid ; the associated porphyritic felspar of the former 

 is often labradorite; that of the latter oligoclase. A rock from Eycott 

 Hill has already been referred to as a labradorite-pyroxene-porphyrite ; one 



(1) Geologische Beobachtungen iiber die vulkanisohen Er^cheinungen, &c. Brunswick 1841. 



