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NOTES ON NORTH OF ENGLAND ROCKS. 



III. 



ALFRED MARKER, M.A., F.G.S. 



It is not a little curious that so little has hitherto been written on 

 the minute structure and constitution of our British calcareous rocks, 

 presenting, as they do, so many points of interest bearing on the 

 conditions under which they were laid down, and the marine-life of 

 the several periods to which they belong. The study is, indeed, one 

 well worth the attention of amateur geologists. The rocks are met 

 with in most parts of the country ; sections of them are easily pre- 

 pared for the microscope ; and their study requires no high-power 

 objectives nor appliances for polarised light. With such a guide as 

 Dr. Sorby's Address to the Geological Society (1879), the composition 

 of the normal calcareous sediments presents few difficulties, and 

 a comparative study of limestones from various horizons and different 

 districts would certainly yield results of interest. 



In the present paper only a few examples will be briefly noticed, 

 and these will be selected from the Oolites of East Yorkshire. 

 The series was named long ago by William Smith, from the 

 prevalence in its calcareous members of the well-known oolitic 

 structure ; but it should be noted that this character is by no means 

 peculiar to the rocks in question, being often developed in the 

 Mountain Limestone, the Magnesian Limestone, and others of very 

 various ages. 



The chief marine member of the Lower Oolites in Yorkshire is 

 the so-called Millepore Oolite. Here the rocks, unless taken from 

 some considerable depth, are always stained yellow or brown, owing 

 to the oxidation of the contained iron compounds. The first 

 specimen is selected from a quarry on the west side of the high-road 

 not far north of Brough [1087]. The oolitic grains, which make up 

 the bulk of the specimen, show very evident concentric coats of 

 brown-stained carbonate of lime, but little or no radial structure. 

 The nucleus about which these concentric coats have been deposited 

 is in most cases a fragment of some organism, such as a chip of 

 a shell, and the various organic remains seen in the slide are 

 invariably covered with a brown coat, even when this is not developed 

 sufficiently to form an ovoid oolitic grain. Many of the shell- 

 fragments are such as might be derived from the valves of certain 

 lamelli branchs. There are also fragments, often of rather oblong 

 form, which consist of a single plate of brown-stained calc ite with 



Naturalisit, 



