Major-Gen. McMahon — Culm-measures at Bucle, Cornwall. Ill 



bichloride of platinum and acetate of uranium, and again obtained 

 unmistakable evidence of the presence of both potash and soda. 

 I also found some lime and magnesia. Iron was absent. 



These results show, I think, that the mica must either be muscovite, 

 paragonite (classed by Tschermak as a variety of muscovite), 1 or 

 margarodite ; but in the absence of a quantitative analysis, the 

 chemical evidence does not enahle us to say positively which of 

 these it is. Margarodite may, however, be eliminated from con- 

 sideration, as, according to Brush, this mineral is decomposed by 

 sulphuric acid, whereas the mica of the rocks under consideration 

 certainly is not. There remain muscovite and paragonite : both 

 contain potash and soda, though in different relative proportions ; 

 but whilst muscovite contains up to 2 per cent, of magnesia and 

 0-5 of lime, these substances, though occasionally present in small 

 quantities, are usually absent from paragonite. 



But though the chemical evidence does not lead to any very 

 definite result, the microscopic evidence gives a clear answer to the 

 question raised. The mica presents the appearance of having settled 

 down with the sand and mud when these beds were originally 

 deposited. It is not arranged in continuous straight lines, or in 

 planes in which mica is abnormally developed, but is scattered 

 promiscuously throughout the rock. In many slices it is oriented 

 in all directions, and even in those which exhibit a laminated 

 structure, the leaves of mica are frequently oriented at variable and 

 considerable angles to the lamination. The leaves of mica are often 

 bent and occasionally ruptured, just as mica would be bent and 

 ruptured if it were deposited in water along with grains of sand, 

 and subsequently compacted by mechanical pressure between angular 

 and subangular grains of such hard minerals as quartz and fel- 

 spar. In this respect the Bude beds have a very close resemblance 

 to the Tertiary Sivaliks of the outer Himalayas. In these mica 

 was deposited along with grains of quartz, felspar, slate, and lime- 

 stone, and the rocks being of comparatively coarse grain, the way 

 in which the leaves were, in the process of consolidation, bent, 

 crumpled, and ruptured, between the grains of quartz, felspar, etc., 

 form an interesting study. 



Schorl and zircon. — The schorl is generally in irregular-shaped 

 grains such as one meets with in granites, but I have observed 

 needles in some quartz grains that may he this mineral. The zircon 

 is in regular crystals such as commonly occur in crystalline rocks. 



Secondary minerals. — Exclusive of vein material, alluded to below, 

 the only secondary substances I have observed are magnetite, ferric 

 oxide, opalescent quartz, carbonate of lime, and magnesite; both of 

 which latter are in a granular condition. 



Veins. — Old cracks in the rocks are stopped in some cases 

 with quartz, and in other cases with carbonate of lime. One slice 

 contains three cracks, which present some features of interest. The 

 one first formed in point of time is a thin one stopped with crystal- 

 line quartz. This was subsequently cut across by a wider crack, 

 1 Third Appendix to Fifth Edition of Dana's Mineralogy, p. 77. 



