LESSONS IN GEOLOGY. 



357 



LESSONS IN GEOLOGY. XV. 



UPPER SILURIAN. 



THE upper Silurian may bo divided into three groups : 

 l.'i il.iw group .... 2,500 feet. 



.'i .iw group . 

 Wenlock group .... 

 Upper Llandorery or May Hill 

 group ..... 



2,500 feet. 

 2,300 



4,000 



The May Hill Sandstone is developed in the Malvorn range, 

 and in its lithological charaotor BO resembles the Caradoo group 

 that at first the two were confounded. Professor Sedgwick 

 first pointed out the mistake ; ho considered those rooks to be 

 the proper base of the upper Silurians. They rest unoon- 

 formably on the beds 

 below them, and are 

 !y distinct 

 from the lower Silu- 

 rians. InthoMalvern 

 hills they exhibit a 

 thickness of some 

 600 feet, and are 

 composed of calca- 

 reous sandstones, 

 which ore nodular at 

 the top. 



From the general 

 and abundant distri- 

 bution of the bra- 

 chiopod, the Penta- 

 merus Icevis (Fig. 39), 

 these beds are some- 

 times called Penta- 

 merus beds. With 

 this fossil another is 

 usually associated 

 the Pentamerus ob- 

 longus (Fig. 42) ; but 

 this latter is by some 

 geologists consi- 

 dered only to be tho 

 young Pentamerus 

 Icevis. As they are 

 wanting in tho 

 groups both above 

 and below, they are 

 very characteristic of 

 these beds ; and as 

 is generally the case 

 when a fossil has in 

 one place a deep sea 

 range, its represen- 

 tatives are very 

 widely distributed. 

 The corresponding 

 Silurian rocks both 

 in America and 

 Russia produce 

 these Pentameri. In 

 Fig. 40 is repre- 



sented a broken fossil of the Pentamerus Icevis, and in Fig. 41 

 the internal cast of the same is shown. Besides these, about 

 sixty specimens of fossils have been found in these beds. About 

 one-half of them extend into other beds of the upper Silurian, 

 and some few are found in the lower. Immediately resting upon 

 the May Hill sandstone is a pale fine-grained slate, which 

 sometimes runs so fissile as to be shaley. Near Tarannon, in 

 Montgomeryshire, the bed reaches the thickness of 1,000 feet, 

 hence it is called the Tarannon shales. It contains but few 

 fossils, and they bear a great resemblance to those of the next 

 surrounding rocks, the 



Wenlock Formation. This group admits of a division, the 

 npper and lower Wenlocks. The upper comprises the Wenlock 

 limestone ; the lower comprises the Wenlock shale, and the 

 Woolhopo limestone. 



Tho Woolhope Limestone is massive and nodular, interstrati- 

 fied with grey shales, and underlying it are fine slabs or flag- 

 stones. Of this bed the mountain ranges of North and South 



Wales are generally formed, and wherever it is found on the 

 surface the aspect is barren and bleak. 



The Wenlock Shale, which soooeeds in the upward order the 

 Woolhope grit, is the most prominent member of the Wenlock 

 formation. It is a thick mass of fine argillaceous deposit, and 

 is worked for flagstones and slates. It contains trilobites and 

 great number of graptolites, together with corals, criaoids, 

 and several other species, which are also found in 



The Upper Wenlock. This bed is a limestone, by no means 

 uniform, bat sometimes concretionary, sometimes thin and 

 flaggy, and often intcntratified with shales. It is exhibited in 

 Shropshire, running some twenty miles from north-east to south- 

 west. The soft shales above and below it hare succumbed 

 to the eroding action of tho rain, and have been gradually worn 



away, leaving the 

 limestone in a ridge 

 prominently above 

 tho surface ; and 

 about a milo distant 

 tho same thing has 

 occurred with the 

 Aymestry limestone, 

 so that these two 

 limestone ridges ran 

 parallel to each 

 other. This lime- 

 stone, as in Lesson 

 Till, we stated was 

 the case with all 

 limestones, was pro- 

 bably built up by or- 

 ganic causes. It is 

 full of fossils, many 

 of them corals, of 

 which there are 

 numerous species ; 

 crinoids with their 

 long stems, and cups 

 and arms. The tri- 

 lobites have several 

 remarkable repre- 

 sentatives, as Caly- 

 mene Blumenlachii 

 and Phacops can- 

 datus, noted for its 

 large size and flat- 

 tened form. Somo 

 of the most promi- 

 nent of these fossils 

 are here figured : 

 Fig. 43, Cyathophyl- 

 him; Fig. 44, JEfelio- 

 Utesi Fig. 45, Gate- 

 nipora; Fig. 4C, 

 Cyathecrinut ; Fig. 

 47, Calymenc Blu- 

 menbachii ; Fig. 48, 

 Phacops caudatus. 



This limeatone.be- 

 fore its geological 



position was fixed, was generally known as the Dudley lime- 

 stone, and the Calymene as the Dudley trilobite. 



We have now reached the topmost of the Silurian formation, 

 The Ludlow Group, which also has its upper and lower divisions. 

 The lower Ludlow consists of a limestone which lies upon 

 certain shales. The limestone is well marked, and from the 

 town near which it is exhibited it is called the Aymestry Lime- 

 stone; the shales beneath have received the name of lower 

 Ludlow shales. They are a dark-grey argillaceous deposit, 

 containing fossils of a higher order than we have found hitherto. 

 In 1859 this bed produced a fish of the genus Pteraspis, which 

 Professor Huxley allies with the sturgeon. This is the first 

 indication of piscine life on our globe, and it is a damaging 

 fossil to tho theory of progression ; for instead of being of a 

 very inferior development, as tho supporters of that theory 

 would have expected, it is in reality a good way up the scale. 

 Of course it may be said that we do not know the whole con- 

 tents of tho lower Silurian beds, and that remains of the lower 



