Crystalline filicide Bocks. 169 



interesting and not unfrequent transitions from slates partly 

 approaching clay-slate, partly chlorite-slate, and partly quartz- 

 slate into Helleflint porphyry (Porphyre a base de Petrosilex 

 of the French), hornstone-porphyry, &c. These porphyries are 

 rendered so much the more instructive by their retaining, even 

 at a considerable distance from the characteristic slate, its par- 

 allel structure to a certain extent, so that the original strati- 

 fication can still be clearly distinguished, although the mass, 

 when examined in small pieces, is a perfectly characteristic 

 porphyry. The same slates pass also into gneiss, so that it is 

 easily explained how strata are sometimes encountered, which 

 consist partly of gneiss, partly of such porphyries together, 

 with complete transitions between the two. Gustav Rose 

 noticed similar transitions in the neighbourhood of Schlan- 

 genberg (Feise nach dem TJraU vol. i. p. 558.) 



2. Each particular kind of crystalline silicide rocks is more 

 especially associated with certain uncrystalline stratified rocks, 

 so that a more or less constant genetic relation evidently sub- 

 sists between two and two kinds of the two great classes : as, 

 for example, between granite or syenite, and clay-slate, be- 

 tween greenstone and greywacke-slate or clay-slate, or newer 

 slates petrographically similar to the clay-slate, between horn- 

 stone-porphyry and flinty-slate, between red porphyry or 

 amygdaloid and sandstone formations, &;c. 



3. It is possible that, in some instances, errors have been 

 committed in the accounts given of the direct occurrence of 

 fossils in the rocks in question, and that, in such examples, the 

 only petrifactions observed were those which had formerly 

 belonged to other older rocks. But, undoubtedly, this has 

 not always been the case when organic remains have been 

 met with in crystalline silicide formations. It is self-evident 

 that such an occurrence must, on the whole, be a rare one ; 

 for the conversion of the original rock into an aggregate of 

 crystals, which is frequently very coarse, must, in most in- 

 stances, have obliterated every trace of the fossils which have 

 previously been more or less perfectly preserved in the mass. 

 However, these occasionally remain, to a certain extent, pre- 

 served, and they then clearly prove that the including mass, 



