356 ROOKS OF IGNEOUS ORIGIN ON DARTMOOR. [Aug. 1 894. 



Weathering often brings out the original structure of rocks when 

 the clean-fractured surface does not betray it, and the Cock's Tor 

 beds are a case in point. The lines of original lamination weather 

 out in a way which imparts to the weathered surface a peculiar 

 ribbed appearance like that of corduroy cloth ; this indicates, in my 

 opinion, that these beds were originally sedimentary rocks deposited 

 in thin layers. This structure is not only apparent in the field, 

 but it is well seen on the weathered surface of one of my hand- 

 specimens, which was selected for microscopic examination and not 

 for the purpose of showing the' surface-ribbing. The illustration 

 (fig. 1) on p. 352, reproduced from a photograph of specimen No. 38, 

 illustrates this fairly well, but the original, when seen in a suitable 

 light, shows the lamination even more strikingly, because the growth 

 of lichen and the consequent variation in tint somewhat interfered 

 with the success of the photograph. 



Evidence of this lamination is not altogether confined to the 

 weathered surface of the rocks ; it can be seen in some of the thin 

 slices under the microscope, as for instance in Nos. 41 and 42. 

 The linear arrangement of the felspathic portions of these slices, 

 when seen between crossed nicols, is precisely that of a fine-grained 

 aqueous sediment, and could be perfectly matched in numerous 

 examples which I possess of the indurated slaty rocks of the Culm 

 series bordering the Dartmoor granite. 



The rocks (38-47) above described are mainly composed of 

 pyroxene and felspar ; they do not contain any water- worn grains 

 of quartz, and their specific gravity averages as high as 3*00. 



The above facts, taken together, seem to leave little, if any, 

 reasonable doubt as to the origin of the rocks, and to show that 

 they were once fine-grained beds of volcanic ash. Their high specific 

 gravity, their mineral contents, and the absence of water-worn grains 

 of quartz put the supposition of their being ordinary sedimentary 

 rocks wholly out of the question ; and the laminated structure, 

 apparent on their weathered surface, is against the notion of their 

 being igneous crystalliiie rocks. Moreover, their internal structure 

 under the microscope is unlike that of any eruptive rock I have ever 

 seen. 



In order to compare these rocks with some of the Lizard schists, 

 I selected for examination specimens taken from a quarry between 

 Landewednack Church and Cove (Lizard). These are typical schists : 

 they still contain some unaltered augite, and are without any free 

 quartz. The specific gravity of the three samples examined was as 

 follows :— (1) 3-00, (2) 3-05, (3) 3-17 (average 3-07). Their specific 

 gravity, therefore, corresponds very closely with that of the ten 

 Cock's Tor specimens. 



I reproduce on the opposite page a sketch of a portion of No. 38 

 from Cock's Tor, as seen under the microscope, and a sketch of one of 

 the Landewednack specimens, above alluded to, for comparison. The 

 structureless portion in fig. 3 represents the felspathic base : the light- 

 shaded part is the augite, and the dark-shaded part the secondary 

 hornblende. I selected that portion of the Lizard slice in which the 



