Correspondence — Mr. A. R. Sunt. 239 



matters : (1) the height to which the mountains must have risen 

 (supposing the mean temperature to have been the same as now) 

 in order that they could have generated glaciers which would have 

 come down to the sea ; (2) whether in this case it is probable that 

 granite would have been exposed at the surface ; (3) whether it is 

 probable (looking especially at the evidence of the breccias in the 

 red rocks of Devon) that high mountains existed in this part of 

 England during Cretaceous times. I may add that Mr. Stebbing, 

 so far as 1 remember his excellent paper, never invoked a Glacial 

 Epoch ; that spectre was raised during the debate ; nor is it 

 necessary to create a glacier to transport the boulders. Shore-ice, 

 such as forms in the estuary of the St. Lawrence, would be quite 

 adequate for the work. If it started from some northern locality it 

 might readily float down (as slab-ice still does in the Atlantic) well 

 south of the latitude of London. T. G. Bonney. 



FOREIGN BOULDEES IN THE CHALK, AND GASES ENCLOSED IN 

 CRYSTALLINE ROCKS. 



Sir, — (1) Referring to Mr. G. A. Martin's interesting suggestion 

 (Geol. Mag., April, 1897, p. 169) that the Betchworth boulder 

 came from Dartmoor, and to its alleged similarity to the 

 granites of Hey Tor and Lustleigh Cleave, the microscope 

 would probably answer this question decisively. Within fifty 

 miles of Hey Tor the floor of the English Channel is strewn 

 with detached blocks of crystalline rocks. Of these blocks I have 

 a large collection of slides, and also a considerable number of 

 specimens from all parts of Dartmoor; collected for the most part 

 by myself, or sent me by the late Mr. R. N. Worth. Not a single 

 specimen of the Channel blocks can be referred to Dartmoor. 

 The minute characters of the Hey Tor and Lustleigh Cleave granites 

 are well marked both positively and negatively. Three of each 

 class of differences will probably suffice — e.g. : presence of fibrous 

 tourmaline ; of chloride inclusions in the quartzes ; of soda felspar 

 in the large potash felspars; — absence of hornblende; of dislocation 

 of minerals ; of any approach to gneissoid structure. Besides these 

 there are others, but most granites not derived from Dartmoor will 

 fail to meet one or other of the above tests. 



(2) In the current number of the Geologica.l Magazine (p. 177) 

 Dr. Tilden's paper on Gases enclosed in Crystalline Eocks is shortly 

 reviewed, and mention is made of hydrogen in the Peterhead granite. 

 On seeing this I referred to some notes made in 1890 on a collection 

 of granites which Mr. Teall had kindly lent me to study. Among 

 these notes occur the following, under the heading " Pink Granite 

 of Eubislaw " ; — "A plane of fracture with fluid inclusions across 

 another with only gas inclusions — order of sequence not made out. 

 Ehombic negatives with bubbles. One negative hexagon with very 

 active bubble. One patch of quartz full of hair-like inclusions 

 and lines of bubbles : in one or two cases the hairs crossing the 

 lines were distinctly divided in two, as seen with -^g. Could find 

 no decided cubic crystal; any sort of crystal (in fluid inclusions) 

 extremely rare. Bending in dark micas." Now it is not too much to 



