Reviews — The Geology of Dartmoor. 173 



the map, the approximate limits of the upland or hill-peat heing 

 indicated by dotted lines, not easy to distinguish at a glance, so that 

 a text-map of the peat-covered areas would have been useful. 



Most of the granite is coarse-grained with large crystals of pale 

 felspar, but some small areas of fine-grained granite, as well as a few 

 felsite or elvan dykes, are distinguished by colour on the map. The 

 bordering ground is occupied by the Middle Devonian Limestone of 

 Ashburton, by Upper Devonian slates with thin limestones, and by 

 Culm Measures. 



These Carboniferous rocks are subdivided into — 



Shale and Grit .... Upper Culm (?). 

 Eadiolarian Chert . . . \ j ^ , 



Shale, Limestone, and Grit j 



The Culm Measures are regarded as " the equivalents of the lower 

 part of the Millstone Grit, and perhaps of the upper part of the 

 Carboniferous Limestone ", and it is considered doubtful whether any 

 of the strata in the district are later than the Millstone Grit. Here 

 we may ask, "What about the Pendleside Series or Upper Limestone 

 Shales?"^ 



In the picturesque region of Holne Chase, as pointed out by 

 Mr. Barrow, the Devonian is found to overlie the Carboniferous, the 

 junction being a plane of overthrust. 



Volcanic outbursts took place both in Devonian and Carboniferous 

 times, and in the former case the rocks described hy Champernowne 

 as schalsteins (Ashprington Series) overlie the Ashburton Limestone. 

 Although mostly "sheared and altered into schalsteins", in some 

 instances the rocks, as observed by Dr. Flett and Mr. Dewey, " are 

 sufficiently well-preserved to show that they had originally the 

 characteristic pillow-structure of spilites." Pillow-lava also occurs 

 around Marytavy, in close association with the Eadiolarian Chert of 

 the Culm Measures. Intrusive sheets of diabase or allied rock are 

 also described as occurring both in the Devonian and Carboniferous 

 strata. Two small dykes of Mica Trap, probably of Permian age, are 

 likewise noted. 



Petrographical descriptions are given of the granite and of the 

 aureole of thermometamorphism surrounding it. Much of the granite 

 is disintegrated and decomposed at the surface, so that it can be 

 dug to a considerable depth with pick and shovel. No china-clay, 

 however, has been found in the area, although it occurs and is worked 

 in the country to the south. It is considered that the " granite 

 probably forms a gigantic laccolite or intruded lake of molten rock, 

 of which the upper surface was no great height above the present 

 surface of the moor". The discussion raised some twenty-five 

 years ago by Mr. Ussher, and carried on by Mr. R. N. Worth, 

 General McMahon, and Mr. A, R. Hunt, is not dealt with, but the 

 Bibliography contains the titles of their papers. To these should 

 have been added the address on "The Geology of Devon", by 

 W. H. Hudleston (Trans. Devon. Assoc, for 1889, and Geol. Mag., 



•^ See Ussher, in Geology in the Field, Jubilee Vol. Geol. Assoc, 1910, 



