500 G. W. Tyrrell—Petrographic Nomenclature. 
apply to intermediate felspathic and felspathoidal types; and 
doleritic applies to the more “ basic” types of igneous rocks corre- 
sponding in general chemical composition with gabbros and basalts. 
With the above use of porphyry the term porphyrite ceases to 
have any significance. It has been used to indicate porphyritic 
hypabyssal rocks of andesitic or dioritic composition. The term 
has been so variously used that it would be better to drop it 
altogether, and to use diorite- or andesite-porpbyry, in which the 
prefixes give the mineral composition, and “porphyry” the 
characteristic texture. 
The terms dolerite and doleritic should always carry the connotation 
of ophitic texture. In some cases it may be difficult to distinguish 
between a coarse-grained basalt and a dolerite (Holmes, p. 83) ; 
but a coarse-grained basalt with its usual intergranular texture 
is quite different from the rock that it is customary to call dolerite, 
at least in Scotland. Moreover, ophitic texture is rare in the 
true effusive basalts, whereas it is very common in the intrusive 
dolerites even when these rocks are of similar grain-size to coarse 
basalts, or of even finer grain. Used by itself the term dolerite 
may be understood to carry the mineralogical implication of 
basic plagioclase, augite, and iron-ore, the rock corresponding 
in chemical and mineral composition to either gabbro or basalt, 
and as being characterized by ophitic texture. The average 
basalt is more “basic”? (mafic) than the average gabbro; and 
corresponding variations are to be found in the dolerites, some of 
which are of gabbroid, others of basaltic composition. ‘This use 
of the term would allow it to be also used, with a textural 
significance, for the hypabyssal equivalents of other “ basic ” 
plutonic and volcanic rocks. There is a real need for terms such 
as teschenite-dolerite, essexite-dolerite, and the like, for rock types 
quite common in Scotland and elsewhere, which can be given no 
more suitable names. 
In connexion with teschenite it should be recognized that this 
term connotes more than “ an alkali-rich variety of analcite-dolerite 
characterized by the presence of idiomorphic purple augite or 
aegirine-augite, and generally containing soda-amphiboles such as 
barkevikite ” (Holmes, p. 224). Many of the rocks are quite as 
coarse as the average gabbro, and are free from doleritic characters 
such as ophitic texture. The term teschenite should denote analcite- 
gabbro, just as theralite denotes nepheline-gabbro. The rocks of 
the smaller intrusions, and marginal modifications of the larger 
and coarser masses do, however, possess doleritic characters; in 
which case they are more correctly referred to as teschenite- 
dolerite or doleritic teschenite. The treatment of teschenite as 
analcite-gabbro has been tacitly adopted by Dr. A. Harker in the 
latest edition of his Petrology for Students (1919), where the rock 
is dealt with in the chapter devoted to gabbros, whereas in the 
previous edition it was described under the heading of dolerite. By 
