6 ROYAL SOCIETY OF CANADA 



filled with deposits of red clay (see Plate III, Fig 3). The varying 

 character and origin of these deposits are discussed below. 



(4) Red conglomerates, with greenish and reddish marls. The 

 thickness of these beds is not definitely known, but is probably not 

 over 100 feet. 



(5) Gray sandstones, with numerous white quartz pebbles. These 

 are the characteristic rocks of the Millstone-grit formation — marking 

 a transition to the brackish and fresh-water deposits of the Coal period. 

 They are found only on the border of the basin and on tlie summits 

 of the higher hills. 



In addition to the above there are heavy beds of red clay, usually 

 resting directly upon the gypsum, with a thickness varying from a 

 few yards to thirty feet or more (Plate V). This clay is very fine 

 and very tenacious. Its colour, due to disseminated anhydrous per- 

 oxide of iron, as well as its character, indicate a very thorough oxid- 

 ation of the materials composing it, with the absence of vegetable 

 matter. Similar clays fill the vertical or oblique fissures, sometimes 

 eight or ten feet wide and twenty or more feet deep, dividing the 

 rock faces of gypsmn or filling irregular cavities in the latter. Occa- 

 sionally they are disposed as more or less horizontal layers between 

 successive deposits of gypsum, but these intermediate bands are more 

 commonly of a gritty nature. Their relation to the coloration of the 

 gypsum is very obvious. 



Confining our attention to the plaster beds we find them to 

 consist, in very variable proportions, of gypsum and anhydrite, their 

 irregular association and the fact that the latter is practically valueless 

 determining the most inijjortant problems in the economical working 

 of the quarries. For many years it was supposed that the workable 

 g}'3)suni beds, especially at the Steves quarry, were underlaid by deposits 

 which were mainly anhydrite or hard plaster, but this has been recently 

 disproved, and it would seem that no definite rule can be ascertained 

 governing the mode of occurrence of the two rocks as regards either 

 horizontal or vertical distribution. In places they are mingled to form 

 a complex network, while in others masses of pure gypsum on the one 

 hand, or of anhydrite on the other, may be found with a thickness, in 

 either case, of thirty feet or more. 



The gypsum itself presents much variety of colour and texture. 

 As to the latter much of the rock is either amorphous or crypto- 

 cr}^stalline ; less frequently it is granular or saccharoidal ; and still less 

 frequently takes the form of selenite. Crystals of the latter mineral 

 are sometimes found irregularly distributed in massive alabaster, but 

 are usually in groups and not infrequently arranged along .lines of 



