Major- Gen. McMahon — Culm-measures at Bude, Cornwall. 109 



bluish grey to the dark colour of ordinary clay-slates. Some 

 are distinctly arenaceous, and may be called fine-grained earthy 

 sandstones ; but they pass gradually into argillaceous shales of a 

 slaty type. No cleavage is appai'ent, but some of the specimens are 

 very distinctly laminated, and one exhibits traces of false bedding. 

 In most of them very minute specks of silvery mica may be observed 

 on the newly-fractured surfaces. Under the microscope these 

 specimens are seen to be made up of quartz, felspar, and silvery 

 mica, with occasional fragments of schorl, a few zircons, more or 

 less decomposed felspathic or kaolinic material, magnetite, ferrite 

 and carbon. Some slices contain calcite, and some magnesite, 

 disseminated throughout them. There are also veins stopped with 

 calcite. The calcite and magnesite, never exhibit twinning. It is 

 evidently a secondary product of aqueous infiltration, and is not 

 an original clastic component of the rock. Fragments of limestone 

 rocks are entirely absent. 



I now pass on to offer a few remarks on the principal minerals 

 found in my specimens. 



Carbon. — The dark lines of lamination which form a marked 

 feature in some of the beds and simulate the fine banding of some 

 crystalline schists, is mainly due to the deposition of carbon, and, in 

 a very subordinate degree, to the presence of magnetite. The pre- 

 sence of carbon was verified in several ways ; by the antimonate of 

 potassa and nitric acid test ; by the application of heat ; and by 

 digestion in hydrochloric and fluoric acids. Prolonged boiling in 

 hydrochloric and in sulphuric acids failed altogether to remove the 

 dark lines from thin slices, but the application of red heat over the 

 Bunsen burner (applied in some cases before, and in others after, 

 digestion in acids) effectually removed these dark bands. 



The presence of carbon appears to be due to the deposit of 

 vegetable matter along with very fine silt. Some of the specimens 

 have obscure markings suggestive of vegetable matter, and one has 

 a small fragment (the rest was broken off in the fracture of the 

 hand-specimen) of what appears to have been the stem of a plant. 



Though the dark lines and bands appear straight to the naked 

 eye, they are seen, under the microscope, to owe their dark appear- 

 ance to discontinuous fibres, and particles, of carbonaceous material 

 distributed in the most irregular and uneven manner among the 

 other materials. The carbon is not confined to the dark layers, but 

 is distributed, though more sparsely, throughout the shales in which 

 it occurs. In some it is very generally disseminated throughout the 

 mass, and the rock has a uniformly dark appearance. Though the 

 carbon for the most part is in threads, or fibres, it is pei'fectly 

 opaque, and does not reveal under the microscope any trace of 

 organic structure. 



Iron. — Iron is present in the form of magnetite, ferric oxide, and 

 apparently in combination with carbon. None of the hand-specimens 

 attract the magnet, but splinters after fusion became magnetic. The 

 iron was evidently deposited along with the carbon. Like the 

 carbon, it is scattered promiscuously through the rock, but is more 

 abundant in the dark carbonaceous bands than elsewhere. 



