The Paragenetie Relations of Minerals. 333 



fluence of alkaline sulphurets, into pyrites which remains 

 imbedded in the clay. 



As the principal object in the present instance is to prove 

 that the formation of imbedded minerals is subsequent to 

 that of their matrices, attention has been especially directed 

 to one mineral alone. There are, however, two others of 

 especial importance which occur in the same manner — 

 gypsum and quartz — the former occurs in sharply-defined 

 crystals in alluvial clays, shale, and even in the schistose and 

 stratified clay iron ore of Yorkshire, &c. The absence of all 

 traces of friction upon these very soft crystals does not, for a 

 moment, admit of the supposition that they were deposited in a 

 detrital manner, or with the matter of the sedimentary strata. 

 It is indeed probable that, in some cases, their formation may 

 have been owing to the presence of iron pyrites, and have 

 taken place in the manner pointed out by Hausmann* 



Quartz being the mineral which principally gives the pecu- 

 liarity of structure to the oldest and most frequent porphy- 

 ritic rock, some mention of its mode of occurrence in more 

 recent rocks may not be without interest. At Posneck 

 (Thuringia), millions of very small and extremely sharp- 

 edged diploheders of quartz occur in a marl belonging to a 

 more recent period than the zechstein group. In a marly and 

 compact limestone at Pforzheim, in Baden, larger crystals 

 are found, which singularly enough contain some sulphur. 

 Quartz crystals likewise occur in the gypsum of Grafintonna 

 (Thuringia), and of St Jago di Compostella (Spain). Even 

 some of the particles of sandstones are not invariably water- 

 worn fragments, but sometimes actual crystals, as in the 

 quader- sandstone of the Tharander Wald (Saxony). 



Another singular fact is the occurrence of small druses of 

 felsite in the clay-stone of Floha (Saxony), together with the 

 same mineral imbedded in a porphyritic manner. This 

 would appear to admit of the inference, that imbedded felsite 

 is, at least in some instances, more recent than its matrix. 



3. It often happens that one mineral imbedded in another, 



* Bemerkungen iiber anhydrite und Karstenit, 1847, p. 25. 

 VOL. LIV. NO. CVIII. — APRIL 1853. Z 



