PRECARBONIFEROUS ROCKS OF CHARNWOOD FOREST. 209 



been studied comparatively, from the less to the more highly altered ; 

 and these again have been constantly compared with rocks, con- 

 taining the above minerals, of undoubtedly clastic character on the 

 one hand, and undoubtedly igneous origin on the other. Taking the 

 case of the felspar first, and confining ourselves to the better-pre- 

 served examples, it is not easy to come to a conclusion — the fact 

 being that it is very difficult to obtain critical characteristics. As a 

 rule, in igneous rocks with a glassy or microcrystalline ground-mass, 

 the porphyritic felspar crystals are rectilinear in outline ; but still 

 rounded, apparently worn, and even broken crystals are not rare ; 

 sometimes also the development of a crystal appears to have been 

 arrested, so that its outline is indefinite and irregular. Of course 

 the frequent inclusion of portions of the matrix in the crystals 

 would be an evidence that they were formed in place; but one 

 cannot argue from its absence. Now to take a few examples from 

 those described above. In such rocks as the banded slates from 

 near Ulverscroft Mill and Bradgate-House Wood (north-east corner) 

 we can hardly doubt, seeing that there is no proof of extreme meta- 

 morphism in their structure, that the included felspar crystals in 

 the coarser bands are of clastic origin ; so also in the comparatively 

 little altered rock from Forest-Gate quarry. But from this we pass 

 readily to the grit of Bawdon Castle and the Woodhouse-Mill rock, 

 and so to the more altered rocks from near the Monastery and 

 Benscliff, from which the transition to the Peldar-Tor and Bardon- 

 Hill rock is not very great. We might further expect that if the 

 rock had been so greatly altered as to allow the formation of crystals, 

 often one third of an inch in diameter, from the base, the latter must 

 have been brought so nearly to a fluid state as to obliterate all trace of 

 its original finer structure, as in the case of the crystallized felspar 

 and mica in a gneiss, where the original constituents have to a large 

 extent entered into fresh combinations, and the new minerals chiefly 

 record the prevalence of the required constituents in those localities. 

 But yet, in the great majority of these Charnwood rocks, we can 

 distinctly make out the general nature of the ground-mass and the 

 included volcanic fragments, with their characteristic structure ; 

 and even in the Bardon and Birchwood slides the ground-mass has 

 not acquired a structure quite resembling that of a true felspathic 

 igneous rock, and in one or two cases gives indications of having 

 once been not perfectly homogeneous. Further, we find that 

 in many much less ancient volcanic ashes broken crystals are 

 largely mixed up with the scoriaceous fragments and amorphous 

 dust — as felspar in the porphyrite tuff of Carlton Hill, Edinburgh ; 

 leucite in the tuff of Borne ; augite and leucite in the peperino of 

 Albano *. We find also that these crystals occur both in minuter 

 powder and in larger fragments, so that there is no antecedent im- 

 probability in those which we have described in these old rocks 



* With which compare some of the augitic or uralitic agglomerate of Rhobell 

 Fawr (N. Wales). Fragments of felspar crystals are also abundant in an in- 

 durated basaltic ash from Caprile (Italian Tyrol), and augite crystals occur 

 among the finer scoria of the Puy Noir (Auvergne). 



