PRE CARB0N1FE ROUS ROOKS OF CHARNWOOD FOREST. 217 



into a very finely crystalline " greenstone," in which faint light red 

 specks can barely be distinguished on a broken surface, though they 

 are a little more evident on one that is polished. About two feet 

 oif the last outcrop of greenstone (or a yard or so more, if that be 

 not in situ) is a large mass of the pecular volcanic grit already 

 described (p. 202), and beyond it a still coarser grit with a band of 

 fine slate. That the former of these is in situ, we have no doubt, so 

 that a junction between sedimentary and igneous rocks is all but 

 exposed here. 



If it had been necessary to separate the northern and southern 

 syenites, we should ourselves, judging from the general character of 

 the Bawdon rock, have classed it with the northern group. It seems, 

 however (and this view is confirmed by microscopic examination), 

 that there is no reason for any such separation, the rock of Hammer- 

 cliff and the coarser variety at Bawdon Castle completely linking the 

 two extreme normal types of Groby and Garendon. The mode of 

 occurrence, the greater change in crystalline structure near the 

 exterior of a mass, and the possibly slightly more basic character of 

 the northern rocks, indicate probably that they were rather less 

 deep-seated intrusions. 



Microscopic Structure of the Northern Syenite. — Specimens of this 

 rock have been examined from the Garendon Quarry, Longcliff, 

 and Bawdon Castle, the first being the most typical. The rock is 

 more affected by decomposition than those which we have been 

 describing, but is one of the same general character. There appears, 

 however, to have been a little less quartz. Yiridite, chlorite, and 

 epidote to a great extent replace the hornblende ; and the usual 

 secondary microliths and kaolin, with more or less of these other 

 minerals, the felspar. So far as can be ascertained, both kinds 

 of felspar have been present. The graphic structure is shown, but 

 is generally more minute than in the other syenites and of the 

 arborescent form. Apatite is not uncommon. The peculiar grey- 

 brown, slightly transparent, aspect of the decomposed grains of iron 

 oxide, and the black lines indicating cleavage-planes, show that 

 much of it is here present in the form of ilmenite. 



The Longcliff specimen is taken from the south end of the ridge, 

 from near a junction; its composition is similar to the above, but it 

 is still more decomposed, and has a rather foliated structure (pro- 

 bably due to pressure), which can be seen in the hand-specimen. 

 It is rather rich in apatite. 



Two specimens have been examined from the boss described in 

 the second wood at Bawdon Castle. One of these, a finely grauular 

 dull-green rock, is from quite the exterior of the boss ; the other is 

 like the Garendon rock, but much more finely crystallized. But for 

 this the latter variety does not seem to differ very materially from 

 that rock, except that the ilmenite seems to be partly replaced by 

 the clear green mineral already noticed (p. 205). The exterior rock 

 is, on the whole, somewhat better preserved ; it has evidently con- 

 sisted of rather long crystals of plagioclase felspar, hornblende, and 

 magnetite or ilmenite. The hornblende has been affected in the usual 



Q. J. G. S. No. 134. q 



