The Precarboniferous Rocks of Charnwood Forest.—Part II

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Abstract
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 sandstone 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 probability 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

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