1849.] 



DAWSON ON THE GYPSUM OF PLAISTER COVE. 



337 



shore at once recedes, and the gypsum occurs at the head of the Cove. 

 The gypsum is well-exposed in a cliff about 80 feet in height ; but, 

 like most other large masses of this rock, it is broken by weathering 

 into forms so irregular, that its true dip and direction are not at first 

 sight very obvious. On tracing its layers, however, it is found to have 

 the same dip with the subjacent limestone and marl. About two-thirds 

 of the thickness of the bed consist of crystalline anhydrite, and the 

 remaining third of very fine-grained common gypsum. The anhy- 

 drite prevails in the lower part of the bed, and common gypsum in the 

 upper ; but the greater part of the bed consists of an intimate mixture 

 of both substances, the common gypsum forming a base in which mi- 

 nute crystals of anhydrite are scattered ; and bands in which anhy- 

 drite prevails, alternating with others in which common gypsum pre- 

 dominates. It is traversed by veins of compact gypsum, but I saw 

 no red or fibrous veins like those of the marl. In some parts of the 

 bed, small rounded fragments of grey limestone are sparingly scattered 

 along layers of the gypsum. 



The exposed part of the mass is riddled by those singular funnel- 

 shaped holes named " Plaister pits," sections of which are exposed in 

 the cliff; they penetrate both the anhydrite and common gypsum, 

 though they are contracted where they pass through harder portions 

 of the rock, and especially the veins of compact gypsum, some of which 

 are only slightly inclined, and look at first sight like layers of depo- 

 sition. The pits of which I saw sections have evidently resulted 

 from the percolation of water through the more open parts of vertical 

 joints, and they were cut off where they were mtersected by another 

 slightly inclined set of open fissures, which afforded a passage to the 

 water. The accompanying sketch (fig. 2) shows one of these pits, and 

 its relations to the joints and stratification of the gypsum. 



(6.) Above the gypsum 



2. 



Fig. 

 Plaister Pits. 



are a few layers of limestone, 

 portions of which appear 

 near the base of the cliff; 

 one of them is studded with 

 tarnished crystals of iron py- 

 rites ; another is a smgular 

 mixture of grey limestone 

 and reddish granular gyp- 

 sum. The portions of lime- 

 stone contained in this rock 

 do not appear to be frag- 

 ments or pebbles, and they 

 are penetrated by plates of 

 selenitic gypsum. They may 

 be parts of a bed of limestone 

 broken up and mixed with 

 gypsum when in a soft state, 

 or the limestone and gypsum 

 may have been deposited si- 

 multaneously and separated by molecular attraction. A rock of this 

 kind is not rare as an accompaniment of gypsum, and it may be 



a. Gypsum vein. b. Open joint. 

 c. Bedding of the gypsum. 



