1864.] T. STERRY HUNT ON LITHOLOGY. 29 



1 



ite), natrolite, iolite, and magnetite are sometimes found as ele- 

 ments in granitic, gneissic, and syenitic rocks. The name of 

 miascite is given to a granitic minature of orthoclase and black 

 mica with elreolite, sometimes with hornblende, albite, and quartz. 



The structure of these orthosite rocks gives rise also to a great 

 variety of names; thus to coarsely lamellar granites the name of 

 pegmatite is sometimes given, while fine-grained mixtures of ortho- 

 clase and quartz have received the names of granulito, leptinite^ 

 and eurite, or when apparently homogeneous and cry j) to -crystal- 

 line are called petrosilex. These latter forms often become porphyr- 

 itic from the presence of crystals of orthoclase, giving rise to or- 

 thoclase-porphyry, or orthophyre. In some of these porphyries, as 

 in those of Grenville, to be described in the third part of this paper, 

 quartz is also present in distinct grains or crystals ; while in some 

 of the red antique porphyries the feldspathic base contains no 

 excess of silica, and occasionally encloses crystals of oligoclase or 

 of hornblende. In many cases the granites, syenites, orthophyres? 

 and other orthosite rocks just mentioned are intrusive; while in 

 other instances, rocks lithologically indistinguishable from these are 

 indigenous, and becoming schistose pass into gneiss and mica- 

 schist. 



The rocks to which the name of trachyte has been given are 

 generally composed in great part of orthoclase (sanidine). The 

 typical varieties of these rocks are white or of pale colors, granu- 

 lar or finely crystalline, and frequently porous or cellular. They 

 appear to consist of grains, crystals, or lamellse of orthoclase, aggre- 

 gated without any cementing medium, and to this seems to be due 

 that roughness to which the rock owes its name. Oligoclase, quartz? 

 hornblende, and mica are also met with in this rock, which becom- 

 ing coarsely granular, passes into granite. Such is the case with 

 the trachytes of the Sierra of Carthagena in Spain, described by 

 Fournet as passing from a dull rough grayish feldspathic mass, 

 into a highly crystalline aggregate of feldspar and mica, with or 

 without hyaline quartz, enclosing hornblende, red garnet, and fine 

 blue iolite. (Comptes Rendus, xliv, p. 1834.) 



The trachvtic texture is not confined to orthosite rocks. Abich 

 has described under the name of trachy-dolerites a group of tra- 

 chytoid anorthosites (dolerites). The cone of the Soufriere of 

 Guadaloupe is described by Deville as a rough granular rock 

 having the external characters of trachyte, from which it is dis- 



