128 REPORT — 1861. 



and leaving the great road from Exeter to Plymouth, on the right, for that which 

 passes over Haldon, in a more easterly direction, to Newton Bushel, reached om* 

 groimd about five miles and a half from Exeter. Mr. Vicary at once pointed out 

 in the red conglomerate one or two well-marked fragments of the true Dartmoor 

 series of rocks, but so far disintegrated that it was impossible to extract them in 

 their integrity ; a farther search was soon rewarded with several less perishable 

 specimens, and amongst them representatives of each of the three kinds of granite 

 described by Mr. Godwin- Austen as occmTing on Dartmoor : there were samples of 

 schorlaceous granite, porphyritic granite, and olvan*. On om* way back to Exeter 

 we detected several well-marked specimens near Peamore, about two miles and a 

 half from the city. 



Though granite pebbles may not have been met with in the conglomerates and 

 sandstones of Torbay, it by no means follows that these rocks are destitute of Dart- 

 moor detritus. Every one who has paid attention to these sandstones must be well 

 aware that in many cases they are eminently micaceous — doubtless a result of the 

 destruction of a large amoimt of pre-existing rock, such as granite, of which mica 

 was a constituent. Nor is it difficult to understand that whilst boulders and pebbles 

 might be unable to force a passage to what is now the South Devon seaboard, com- 

 paratively small thin flakes of mica might succeed in accomplishing the journey. 

 We may have here an indication that the direction of the prevailing and powerful 

 currents was not eastward, but north-east and northerly — not from Dartmoor to the 

 coast of South Devon, but towards Haldon and North Tawton. 



The gTanites of Dartmoor, then, are limited in age, on the side of antiquity by the 

 culmiferous beds of Devon, and on tlie modern side by the red conglomerates : 

 what is the place of these in the chronological scheme of the geologist ? The answer 

 has long been given respecting the first : " The upper division of the culmiferous 

 beds contains fossils identical with those in the upper division of the coal-measuresf." 

 But the age of the conglomerates is less easily determined. That they belong to the 

 New Red Sandstone there can be no doubt, since they are above the Upper Coal- 

 measures and midcrlie the Lias ; but whether Upper or Lower New Red, tliat is, Tri- 

 assic or Permian, is not so certain as could be wished. They are entirely destitute of 

 fossils, excepting such only as occm- in the pebbles. The sandstones are evidently 

 of littoral origin ; their surfaces frequently display fossil sea-ripples, sun-cracks, and 

 impressions of rain-drops ; but no footprints or organic traces have ever been detected 

 on them ; there are no palseontological indications of their age. 



More than one eminent geologist has been struck by the angular character of 

 the fragments composing the conglomerates (more correctly breccias), and has 

 remai'ked that, in its pliysical character and general appearance, the formation is 

 rather Peimian than Triassic. It is, as is well known, coloiu-ed in our geological 

 maps aa on the horizon of the Lower Trias. The granite pebbles of Haldon may 

 perhaps go far to confirm this decision. 



Whatever may be our opinions respecting the origin of gi-anite, whether we hold 

 it to be a strictly igneous or a thermo-aqueous product, an original or a SUperim- 



5>osed phase of rock existence, there is probably no doubt that it was formed in 

 ^lutonic depths, a hypogeuo formation requiring enormous pressure for its elabo- 

 ration. 



Mr. Sorby estimates the pressure under which the St. Austel granite was formed 

 as equivalent to that of 32,400 feet of rock %. He gives no estimate for Dartmoor, 

 but we shall probably not exceed the truth in taking this, his lowest Cornish estimate, 

 which gives us a pressure equivalent to that of a pile of rock six nules in thickness ; 

 but as the pressm'e was probably due to the expanding power of some agency acting 

 beneath or within the granitized mass, requiring resistance and not pressure, 

 strength and not weight in the overlying crust, we may content ourselves with a 

 small fraction of this. Still there must have been a crust of very great thickness at 

 and after the close of the Carboniferous period, or the granitic form could not have 

 been assumed by the mass beneath ; and this crust must have been stripped off and 

 the granite laid bare before the era of the accumulation of the red conglomerates, or 



* G-eol. Trans, vol. vi. pt. 2. p. 477. 



t Prof. Sedgwick and Sir E. I. Murchison in Geol. Trans, voL V. pt. 3. p. 687. 



X Quart, Joiuru. Gkol, Soc. vol. xiv. p. 494. 



