368 ME. H. H. ARXOLD-BEMEOSE OX [Aug. I904, 



The rock is similar to the quartzite that occurs in the" dykes in 

 the quarry. Slides 1238 & 1239 consist of quartz in a mosaic of 

 granitic structure, with a small quantity of mica and pebbles 

 of microcrystalline quartz (see PI. XXXI, fig. 5). The laminated 

 quartzite (1249) shows the laminations better in a hand-specimen 

 than under the microscope, and contains more mica than 1238 & 

 1239. A thin slice of Nuneaton quartzite, compared with Marston 

 rock, was found to contain larger grains of quartz, but in other 

 respects to have a similar structure. 



The failure to find water was probably because the sandstone- 

 grains were cemented by secondary silica, which had rendered the 

 rock impervious to water. 



About 800 feet south of Marston-Common Farm is an old sand- 

 stone-quarry, on ground mapped as Keuper Marl by the officers of 

 the Geological Survey. From its position and from the fact that 

 the ground-surface is lower than that at the farm, we may infer that 

 the sandstone-beds are probably on the same horizon as the quartzite 

 found in the well. Two thin slices of this rock were examined 

 (1319 & 1320). They are similar to some of the quartzite-dykes in 

 the quarry. The roc]* consists mainly of an aggregate of quartz- 

 grains, with a small quantity of mica, and perhaps of felspar. 

 Some grains consist of microcrystalline quartz. (See PI. XXXT, 

 fig. 6.) 



V. Conclusions. 



The Snelston inlier consists of massive beds of limestone with 

 occasional nodules of chert, and is unaccompanied by shales : it must, 

 therefore, belong to fhe main mass of the Mountain-Limestone, 

 though separated from it by a large synclinal basin. The quartzite 

 in the dykes is similar to the Keuper Sandstone in the immediate 

 neighbourhood of the limestone-inlier. It requires no great stretch 

 of imagination to suppose that the limestone, traversed by joints and 

 fissures, was covered by water in which the Triassic sandstones were 

 laid down. The angular and rounded grains of quartz, with the few 

 felspars and fragments of mica, were probably deposited in these 

 fissures, and solidified as dykes of sandstone. At a later period, the 

 silica was introduced which cemented these sandstone-dykes and 

 the sandstones at Marston-Common Farm into a quartzite, and 

 impregnated the limestone in such a way as to form a quartzose 

 limestone similar to the quartzose limestone near Bonsall, Castleton r 

 Ashover. and in other parts of the county. 



EXPLANATION OF PLATES XXX & XXXI. 

 Plate XXX. 



Quartzite-dykes in Mountain-Limestone near Snelston (Derbyshire). 



Fig. 1. Four-inch dyke of quartzite, passing through the limestone in a vertical 

 direction near the centre of the figure! 

 2. Larger dyke of quartzite, 10 to 12 inches in diameter. The quarry-face 

 is a slickensided surface. 



[Both figures represent an almost vertical face of the quarry -Mall. J 



