ON THE PRECAR30NIFER0US ROCKS OF CHARNWOOD FOREST. 199 



18. The Precarboniferous Rocks of Charnwood Forest. —Part II. 

 By the Rev. E. Hill, M.A., F.G.S., Fellow and Tutor of St. 

 John's College, Cambridge, and the Rev. T. G. Bonne y, M.A., 

 F.G.S., Professor of Geology in University College, London, 

 and Fellow of St. John's College, Cambridge. (Read January 9, 

 1878.) 



[Plate X.] 



Contents. 



The Microscopic Structure of the Clastic Rocks of Charnwood Forest. 

 The Igneous Rocks of Charnwood Forest. 



(1) Preliminary. 



(2) The Southern Syenite. 



(3) The Northern Syenite. 



(4) The Granite of the Quorndon District. 



(5) The later Intrusive Rocks. 



Probable Outliers of the Charn wood-Forest Eocks. 

 Faults of the Forest region. 

 Age of the Clastic Rocks. 

 Age of the Igneous Rocks. 



The Microscopic Structure of the Charnwood Rocks. 



In many cases the task of determining the nature and structure of 

 these rocks is comparatively easy; but in others it presents great 

 difficulties. These arise from the amount of metamorphism which 

 the rocks have undergone since they were first deposited. This 

 metamorphism may be said to be of a double nature : — the one, 

 readily apprehended by the eye, that which has converted sand- 

 stone into quartzite, and fine felspathic detritus into something 

 resembling a felspathic igneous rock, being in all probability the 

 combined effect of pressure, heat, and water ; the other, almost 

 wholly revealed by the microscope, in which the last of the above- 

 named agents of change has probably been the most active, and 

 which may still be in progress — namely, the gradual decomposition of 

 the minerals which once composed the rocks, and the formation of 

 new ones by fresh combinations of the chemical elements thus set 

 free. This process often obscures, far more than the former, the 

 original structure of the rock ; and it is this accordingly which 

 causes our main difficulty in the study of these Charnwood rocks. 



To investigate microscopically all the varieties of stratified rocks 

 found in the Forest would be a most laborious task ; and in all pro- 

 bability the result would not compensate for the labour. We 

 have therefore selected a series of specimens which appeared either 

 typical of the more important varieties, or likely to be useful in 

 illustrating some point of structure or stratigraphy. Forty-four 

 slides in all have been examined*, which may bo thus classed — (1) 

 Quartzites, (2) Slaty Rocks, (3) Coarser Fraginental Rocks, (4) 



* Prepared very skilfully by Mr. F. G. Cuttell, of 52 New Coinpton-street 

 Soho. 



