416 



THE GEOLOGIST. 



see Avliy tlie theory of the " non-protusion" (if I mav so call it) of gi'aiiitc 

 shoiilcl not be regarded as very plausible ; but if, on tlie other luuid, instances 

 have been discovered, then I sLould think the idea must fall to the ground, 

 unless the facts can be accounted for in any other reasonable manner. 



I am but a young geologist, and therefore not thoroughly versed in all the 

 details and intricacies of this most interesting science ; but this is a subject 

 upon which I should be very glad to obtain some information — Yours, &c., 

 Edmund St. Aubyn. — Granite has always been regarded by modern geologists 

 as a crystalline rock formed under great pressure in the depths of the earth. 

 Mr. SoVby has written some excellent ]3apers on the evideiice of the presence 

 of heated water in the changes effectuig the transnuitation nito granite ; and in 

 the elementary w^orks of Sir Charles Lyell and others, diagrams are given illus- 

 trative of the manner of the metamorphosing action, which display also how 

 ihe lowermost or deepest granite must be the most recently formed. 



No evidence wdiatever, so far as ^ve know, exists of the over-run or out -flow 

 of granite like a lava-stream. Such efiluxes of volcanic matter are found in the 

 form of bedded trap-rock, basalt, &c., all of which have been ejected from an 

 orifice or chunney of eruption. Not so granite, which is boss-like and, probably 

 even, only ordinary sedimentary rock changed or altered by the uprise of the 

 range of the isothermal lines of the internal heat consequent on the stopping 

 out, by the deposition in the ancient ocean-basins of thick masses of sedhnent, 

 of the conductive action of the ocean-water on the principle pointed out in the 

 article on "Common Eossils," at page 154 of the present volume of this 

 Magazine. 



The action of heated water combined with pressure is accounted for in the 

 consequent heating, under such circumstances, of the infiltered water always 

 met with at great depths. The principal evidence brought forward by Mr. 

 Sorby on this point is the presence in granite-rock of small cavities partly fiUed 

 with water, the explanation of which is, that being originally bubbles of hot 

 vrater or steam, as the cooluig of the granite took place, these contracted in 

 dimensions, leaving the cavities only partly filled with globules of the con- 

 densed fluid. 



The continued heating and expansion of the lowermost rock-masses or other 

 causes and actions may have caused a protrusion of the upper and solidified 

 portion of a granite-mass, m some rare or doubtful cases, but such a fact would 

 in no way militate against the general doctrine of the graduality and pro- 

 fiuidity of the granitizmg operations. 



Formation or Mineral Veins by Simple Sedimentary Deposit.— Sir, 

 — The excellent paper on the deposition of strata in your April number, show- 

 ing hoAV umiecessary it is to refer to any other cause than the natural shoal 

 formations, the difl'erent complicated appearances of horizontal and perpen- 

 dicular strata leads me to ask if mineral-veins have not had the same origin ? 

 I mean, have they not been laid down in fine seams perhaps on sloping shoals ere 

 t hey or the beds containing them were fused and crystallized by volcanic agency ? 

 lu-om whatever source the minerals themselves were derived — and we know 

 lliat silver, copper, &c., are finely diffused in the sea — there is nothing impro- 

 bable in the idea that they may have been strewed over a muddy or sandy 

 shore, as the case maybe; and no one can walk along a rock interstratificd with 

 (juartz, (Src, a\ iihout coming to the conclusion that it in this way came into its 

 ])resent position. The ores and A^arious metals they contain a])pcar to me to 

 liave been ;ilike (h^])osited in thin layers at intervals amongst the other debris. 



M.'iy I also tro;il)le you to teU me what you consider the best authority to 

 consiih upon lul'uso;ia ? Also on the formation of fossils, as 1 bcHeve the cir- 

 cmnsliinct's iiu(h'r wliicli \\\q latter were produced must have been extremely 

 rare: loi- iusiaiu'c, liad the slioal of Sand-launces mentioned by Dr. Dawson 



