Vol. 60.] QUARTZITE-DYKES IX rUOEXIAIX-LIWESTOXE. 367 



III. Silica peesexi ix two Forms. 



From the foregoing description, it appears that the silica is present 

 in the limestone in two forms, which have had an entirely-different 

 origin. The one, similar to that iu the quartzose limestone previously 

 described by me as occurring in various parts of the ^Mountain-Lime- 

 stone area of the county ; and the other associated with felspar and 

 mica, sometimes forming a calcareous grit, at others a quartzite. 

 In the former case, the quartz occurs in isolated crystals and crys- 

 talline aggregates and in small veins or strings in the limestoue ; 

 in the latter, it occurs in dyke-like masses, which mainly consist of 

 detrital and angular grains. 



It may be convenient to refer briefly to sandstone-dykes which 

 have been previously described. The references to them have been 

 obtained from Sir Archibald Geikies ' Text-Book of Geology,' 4th 

 ed. vol. i (1903) pp. (365-67 : — 



In Ross-shire narrow rifts or cracks in Lewisian Gneiss have been illled 

 with Torridonian conglomerate and sandstone. 



(b) Dykes of hard fossiliferoua sandstone traverse the Xeocomian clays of 

 Alatyr, in Russia. These clays are supposed to have been rent open by a 

 submarine earthquake, and filled up with deposits from the sea-floor. 



In Colorado a series of sandstone or quartzite-dykes traverse a pre-Cambrian 

 granite. Mr. TV*. O. Crosby suggests that the fissures were formed at the time 

 of the production of the great fault of Ute Pass, and that they svere filled 

 with sand from the overlying Potsdam Sandstone. 



\d) In Northern California Mr. J. S. Diller found dykes of impure quartzose 

 sandstone intersecting Cretaceous sandstones and shales along lines of joint, 

 and suggested that they represented earthquake- fissures filled in with sand 

 rapidly injected from below. 



(e) Mr. E. Greenly described some sandstone-pipes in limestone in Anglesey, 

 descending from a bed of sandstone into a limestone. 



IV. Origin of the Quartzite-Dykes ix the 



LlVESTOXE-lXLIER. 



The detrital form of the quartz-grains and the slight traces of 

 bedding seen in one of the dykes indicate that the quartz, mica, 

 and felspar were introduced into the limestone-fissures from above. 

 According to the Geological-Survey Map, the Keuper Marl rests upon 

 the limestone in the neighbourhood of the quarry. The sections seen 

 in the quarries seem to indicate that this mapping is correct. In 

 trying to find an explanation of the origin of these quartzite-dykes 

 in the limestone, I examined the neighbourhood of the quarry for 

 sections of Keuper rocks in the year 1901. At Marston-CommonFarm, 

 1 200 yards south-west of the quarry, I found that a well was being 

 sunk for water. It was started in Keuper Marl, went through 8 or 

 10 yards of it and 21 yards of a very hard grit or quartzite, which 

 was sometimes in thin lamina? and at others contained small pebbles 

 of quartzite. At the time of my visit, the work had just been 

 abandoned, because of the absence of water. The information and 

 measurements were obtained from one of the men who were engaged 

 in the work. I made a selection of specimens of the quartzite from 

 the sinking, and examined several thin slices. 



