LEONHARD ON PORPHYRY. 



phyries. To these belong certain Thuringian porphyries with Httle 

 or no quartz, and which B. Cotta calls Glimmer-Porphyre (Micaceous 

 Porphyry). "I must again repeat," he says, "that much of that 

 which is generally called Melaphyre probably belongs to the Glim- 

 mer-Porphyre, which is often vesicular and amygdaloidal." 



One of our first mineralogists has lately published a very in- 

 structive memoir " on the rocks belonging to the granitic group *." 

 Both Porphyry and Syenite-porphyry are included in this group. 

 Porphyry, according to G. Rose, consists of felspar, oligoklase, 

 quartz, and mica, which form a compact matrix containing large cry- 

 stals of the same constituent elements. Syenitic porphyry contains 

 in a compact paste crystals of felspar, mica, oligoklase, and horn- 

 blende. It is only distinguished from the former by not containing 

 quartz (or at least very seldom). 



Naumann, in his ' Lehrbuch der Geognosie,' adopts another view. 

 According to him, the felsitic porphyries form a separate group. This 

 includes all those porphyries the paste of which appears to be a 

 close and fine-grained mixture of felspar and quartz, which melts 

 under the blowpipe. This paste or matrix is called felsite by Nau- 

 mann, and the porphyry is called felsite-porphyry. The paste con- 

 sists chiefly of the following minerals : — felspar (orthoklase) in grains 

 or crystals ; oligoklase and sometimes albite ; quartz in grains or cry- 

 stals ; and mica. Amongst these the quartz is by far the most im- 

 portant, inasmuch as the distinction of " quartziferous" or " non- 

 quartziferous" depends on its being present or not. With reference 

 to these distinctions, Naumann admits the following rocks into the 

 family of the Felsite-porphyry : — 



1 . Non-quartziferous Porphyry. The usually dark paste contains 

 crystals of felspar, mica, and sometimes hornblende ; quartz is alto- 

 gether wanting, or only seldom occurs. 



2. Minette. A rock consisting principally of mica. 



3. Granitic Porphyry. In the composition of the paste, besides 

 quartz and felspar, mica and perhaps a little chlorite appear to con- 

 cur. The paste contains crystals of felspar, oligoklase, quartz, mica, 

 and sometimes scales of chlorite. 



4. Felsite-Porphyry (Quartziferous Porphyry). Quartz is always 

 found in the paste or matrix, two kinds of felspar generally occur, 

 and sometimes mica also. 



5. Pitch stone-Porphyry. 



It was the original object of this memoir only to describe the oc- 

 currence and conditions of the quartziferous porphyry. The author, 

 however, found that this was not altogether practicable, on account 

 of its close connection with some of those porphyries that contain 

 little or no quartz. He therefore includes these also in his remarks ; 

 and, chiefly following Naumann' s classification, gives to the group 

 the general term '* Felsite-porphyry." This, according to its mine- 

 ralogical characters, consists of three subdivisions; — 



1. Quartziferous Porphyry. 



2. Granitic Porphyry. 



3. Non-quartziferous Porphyry. 



* G. Rose, Zeitschrift d. dcutsch. geolog. Gesellsch. vol. i. p. 352 et seg., 1849. 



