Vol. 57.] SILURIAN [?] ROCKS IN" FORFARSHIRE, ETC. 333 



larian ooze is by no means so satisfactory as one could wish. Still, 

 if we compare the hand-specimens and microscopic sections with 

 those of the cherts from the Southern Uplands, where the latter 

 are somewhat altered, little doubt can be entertained that the cherts 

 in these widely-separated regions had a common origin. 



As already indicated, the term Green Hock is applied to a 

 variable series of basic igneous rocks, usually green, a coloration due 

 to the presence of a green mineral, which is often chlorite. lu 

 many places, however, it is a more or less distinctly green material, 

 generally isotropic, which may be conveniently named viridite. 

 The rocks vary considerably in texture, and to a less extent in 

 mineralogical composition, and, with one local exception, the 

 variation in ultimate chemical composition is small. The most 

 abundant rock is an ophitic dolerite, in which the original basic 

 mineral was augite. Indeed this type differs from the central part 

 of a broad dolerite-dyke of Tertiary age solely in the fact that the 

 Jatter contains more iron-ore than the Green fiock. The variation 

 in the coarser varieties consists mainly in the partial replacement 

 of augite by original brown hornblende and brown mica. This 

 change is often accompanied by a slight difference in structure, the 

 type containing these two minerals being not so markedly ophitic. 



The coarseness of texture suggests that these rocks are probably 

 intrusive. Indeed, in many cases it is clear that the igneous rock 

 becomes more fine-grained at its point of junction with the jasper ; 

 but there is still a considerable residuum of fine material too de- 

 composed to enable us to say confidently that it is only the quickly- 

 cooled edge of an intrusion, and not part of a lava-flow. The 

 junction-line with the jasper is often incrushed, and notwithstanding 

 decomposition, it is evident that the upper surface of the igneous 

 mass was not originally slaggy or uneven. The jasper has not been 

 found filling cracks or fissures in the igneous rocks, as recorded in 

 the case of the radiolarian chert in the Southern Uplands. 



In several localities curious au gen-shaped cavities in these 

 basic rocks have been observed, suggestive of a coarse vesicular struc- 

 ture. These are so numerous at one locality near Brawliemuir, that a 

 specimen showing one of these cavities was sliced in order to ascer- 

 tain its character. This section (5570) showed that the hollow 

 was formed at the intersection of two crush-planes, where the lime 

 set free by crushing had been accumulated, and this being dissolved 

 out afterwards, a cavity has been left. 



Close to the locality just mentioned some pale-green and white 

 igneous rocks are met with, which contain much more felspar 

 than the normal Green Rock, and seem to have possessed an original 

 flow-structure. They are more suggestive of a lava-flow, but they 

 are so crushed and decomposed that it is impossible to determine 

 precisely their original characters. 



The Green Eocks, as a whole, seem to consist of a series of sills 

 of ophitic dolerite, varying slightly in mineral, composition and 

 structure, and rarely continuous for any great distance. The fine- 

 grained material is probably the quickly-cooled edge of these sills 



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