Piirbeck Beds. 203 



Purbeck series, is sudden. These are about 8 feet thick, 

 and contain fresh-water remains of the genera Cyclas, 

 Valvata, Limncea, Physa and Cypris. 



Near the base of the Purbeck rocks, in the Isles of 

 Portland and Purbeck (figs. 39 and 75), lie three beds, 

 known as the 'dirt beds,' which, from their colour 

 and earthy character, were clearly ancient soils. They 

 are full of the silicified stems and stools of coniferous 

 trees, the former procumbent and the latter with roots 

 attached, standing in the soil in the position in which 

 they grew. Plants (Cycadites microphyllus, &c.) 

 allied to the modern Cycas and Zamia are also found 

 in them. In the Isle of Purbeck the whole of the Pur- 

 beck strata are about 360 feet thick in their largest 

 development. They consist chiefly of limestones and 

 marls, principally of fresh-water origin, with inter- 

 stratifications of marine, brackish-water, and terrestrial 

 bands. According to the sections of the Government 

 Geological Survey, by Bristow, there are indications of 

 four terrestrial surfaces, eleven sets of fresh-water beds, 

 four brackish water, and three marine bands, the last 

 containing Pectens, Modiolas, Aviculas, and Thracias. 

 One of these, the ( cinder-bed ' of the quarrymen, is 

 about 1 2 feet thick, and is composed almost exclusively 

 of oysters (Ostrea distorta). Along with these, spar- 

 ingly, was found a Perna and an Oolitic genus of Hemi- 

 cidaris, H. Purbeckensis. The fresh-water shells of 

 the various beds are chiefly species of Paludina, Lim- 

 ncea, Planorbis, Physa, Valvata, and Unio, and 

 Cyclas, and along with these are several species 

 of small fresh-water bivalve Crustacea of the genus 

 Cypris. The celebrated Purbeck marble, so largely used 

 in the palmy days of Gothic architecture for the decor- 

 ation of churches, lies near the top of the Upper Pur- 



