250 



occurs chiefly in sheets among the mica-schists and quartzites, contorted 

 and crumpled with them, and often apparently resembling only altered 

 beds of the metamorphic series. Their intrusive nature, in most cases, may 

 however, be determined on careful examination by the cutting across of 

 some beds in part of their course. . . . These sheets and 

 dykes, for the greater part, seem to have been intruded among the sedi- 

 mentary strata, from which the metamorphic rocks have been formed, 

 previous to the occurrence of the metamorphic action, and to have been 

 displaced, contorted and themselves metamorphosed with these beds. . . 

 It is of frequent occurrence to find in thick sheets of this 

 trap that the central portion is apparently a micro-crystalline basalt 

 (melaphyre), passing by imperceptible gradation into a fibrous hornblendic 

 rock, and this again, towards the margin of the sheet, into a black 

 micaceous schistose mass in platy layers, conforming to the bedding of the 

 adjacent mica-schists. . . . They [the rocks] are composed of 

 plagioclase, the triclinic characters of which are clearly visible, and a 

 pyroxenic mineral which probably owing to the metamorphic action is 

 variable, augite, hornblende, and diallage graduating into each other. Green 

 chlorite seems also to be largely present, in fact to form almost an essential." 

 In the absence of detailed petrographical descriptions it is impossible 

 to speak positively as to the characters of the above rocks. It seems 

 probable, however, from the above descriptions that we have here illustra- 

 tions of the development of hornblendic and chloritic schistose rocks in 

 consequence of dynamic metamorphism operating upon intrusive sheets. 

 The date of the dynamic metamorphism has not been actually 

 determined. It was certainly pre-Carboniferous because pebbles of the 

 metamorphosed rocks occur in the basement Carboniferous conglomerate. 

 The general strike of the metamorphic series (mica-schists, quartzites, 

 etc.) in which the igneous rocks occur is approximately N. and S. It 

 seems probable therefore that the metamorphism is of the same age as that 

 of the north-west of Scotland (post-Ordovician). 



So far we have been referring to rocks in which the dominant ferro- 

 magnesian constituent either is now or was originally some variety of 

 pyroxene. We have, in the next place, to refer to basic rocks in which 

 hornblende plays an important part as an original constituent. Such rocks 

 are usually termed diorites. Some of the more granitoid rocks have been 

 termed hornblende-gabbros. 



Basic plagioclase-hornblende rocks have been recognized in Leicester- 

 shire, Warwickshire, Anglesea, the Lake District and the Highlands of 

 Scotland. The Warwickshire rocks were described by Mr. ALLPORT w in 

 1878. They vary considerably in texture, mineralogical composition and 

 state of preservation. They are associated with Cambrian strata into which 

 they have been intruded, mostly in the form of sheets. The original 

 hornblende of these rocks occurs as well defined brown prisms or as deep 

 brown ophitic masses, often giving rise to green and even colourless 



(1) Q. J. G. S. Vol. XXXV. p. 637. 



