170 



HARD WICKE 'S S CIENCE - G O SSIP. 



group, is still a much-vexed question.* The age 

 and exact equivalents of the several subdivisions of 

 the Triassic rocks have not been established ; and 

 the Greensand hills of Haldonare outlying fragments 

 of the Blackdown beds, the subject of great discus- 

 sion. Confining ourselves as much as possible to 

 facts, we may take a section from Dartmoor to 

 Plymouth, as drawn by Sedgwick and Murchison, 

 which gives the general structure of that line of 

 country, t 



The granite, which forms the highest ground, is a 

 pale grey, or white, porphyritic rock, containing large 

 crystals of felspar J and it has been thrust up amid 

 the Paleozoic rocks, and even intruded as veins 

 amongst the slates, so as to produce great meta- 

 morphism in its immediate vicinity. It has burst 

 through both Devonian rocks and Culm-measures,^a 

 very significant fact, and one which sufficiently ex- 

 plodes the early notion that granite is always the 

 oldest rock. Thus the granite of Dartmoor is more 

 recent than the Culm-measures, but whether these 

 rocks represent the whole of the Coal-measures, or 

 merely the lower part, is a question that has yet 

 to be settled. We have the means, however, of 

 marking off the age of the granite in another 

 -direction. Large boulders of this rock are oc- 

 casionally met with in the Triassic rocks near 

 Teignmouth. Hence it must have been formed in 

 an earlier period, and may very likely be, as has 

 been suggested, of Permian age. 



In our section, the granite abuts against the meta- 

 morphosed Devonian slates ; these assume their 

 natural character of bluish-grey and claret-coloured 

 slates further south. In them are occasionally found 

 various igneous rocks, most of which, according to 

 Mr. Worth, are contemporaneous with them. They 

 pass beneath the limestone of Plymouth, which rests 

 conformably upon them. 



The Devonian limestone, which is so conspicu- 

 ously developed at Plymouth and in the cliffs that 

 face the Sound, is a bluish-grey crystalline rock, 

 sometimes stained red, and veined with calc-spar. 

 In its general aspect, and in the scenery it produces, 

 it reminds us forcibly of the mountain limestone ; 

 but when we come to study the organic remains, it 

 will be found that, especially in its coral fauna, the 

 forms of life were different. From the southern 

 portions of Plymouth this limestone stretches some 

 two or three miles eastward of Oreston, and it is again 

 developed in the neighbourhood of Yealmpton. A 

 study of the geological maps of this district, and of 

 that around Torquay, would seem to indicate that 

 the limestone occurred in great lenticular masses. 

 But although in places the limestone becomes more 

 or less shaly, and has been considered to pass 



This question has recently been reviewed by Mr. R. N. 

 Worth in an article on the Geology of Plymouth,— r^rtwi. Ply- 

 Jiwuih Inst., vol. v. p. 450. 

 t Trans. Ccpl. Soc, 2nd Ser., vol, v. Plate LI. 



almost directly into this type of rock, yet a careful 

 study of portions of the limestone district near 

 Newton Abbot convinced me that its frequently 

 abmpt termination \<2.% more often due to faults 

 than to any disappearance of the limestone in its 

 passage into slates. 



In that district we find a well-marked succession (in 

 descending order) of— (3) Limestone, (2) Slates,'and 

 (i) Red Sandstones, very like Old Red Sandstone; 

 and the same divisions have been very carefully 

 mapped out in the countiy around Totnes by Mr. 

 Champeraowne. 



Now, in their section drawn from Plymouth to 

 Bolt Head, Sedgwick and Murchison i^epresented a 

 series of contorted red sandstones as abutting 

 against the limestone of Mount Batten, Plymouth, 

 and their diagram would make them appear to rest 

 upon it. They state that this sandy division, "in 

 many parts, is exactly like the Old Red Sandstone." 

 It is quite possible that the southern margin of the 

 Pl3'mouth limestone may be a faulted one, as sug- 

 gested to me by Mr. Champemowne ; or the structure 

 may be that of an inverted anticlinal, as supposed by 

 Jukes. The red sandstones may therefore be on the 

 same horizon as those which occur at the base of the 

 slaty rocks before mentioned, and which are very 

 well exposed in a quariy at Cocklngton, near Torquay. 



The red sandstones of Staddon are overlaid by 

 greenish-grey and sometimes "glossy" slates, which 

 occasionally yield slates useful for roofing purposes, 

 and these are stated to occur in planes parallel to the 

 bedding. 



In the promontory of Bolt Head and Salcombe, the 

 beds have been highly altered into micaceous and 

 slightly chlorltic slates. No direct clue to the agent 

 which produced this change can be seen ; but Jukes 

 was of opinion that a boss of granite may be approach- 

 ing the surface in this region, and perhaps reaches it 

 under the sea in adjacent parts of the Channel.""' 



Although fossils are not common in the slaty series, 

 specimens o{ Spirifcr, Ortliis, Lcptcnia, and Trllobites 

 may sometimes be procured. 



In the Plymouth limestone, and in that developed 

 around Torquay and Newton Abbot, many beautiful 

 fossils have been obtained. But it is generally 

 necessary for the specimens to be polished before 

 their structure can be well seen, and the visitor may 

 frequently be disappointed In his search among the 

 quarries. He must, however, look out for the 

 weathered surfaces of the rock, and as the fossils are 

 better capable of withstanding the wear and tear of 

 atmospheric agencies than their matrix, they may not 

 unfrequenlly be found standing out in bold relief. 



Wherever the limestone is developed, quarries 

 abound, for the stone is extensively dug for building 

 and paving jaurposes, for road-mending, to be burnt 

 for lime, or to be polished for purely ornamental uses. 



* " Notes on parts of South Devon and Cornwall, ' 1868. 



