92 Correspondence — Mr. J. R. Gregory. 



not deep, and runs over a bed of stones, mostly small boulders 

 of Trap, from three to six or eight inches in diameter. Living 

 examples have long been known from the Eiver Nile, and there 

 are also some specimens in the British Museum having the 

 locality of Natal attached to them ; but I believe these are the first 

 that have been found in Cape Colony. Between Natal and Cape 

 Colony are the Drachenberg and other ranges of mountains, yet this 

 species of Cyrena seems to be the same frOm both localities. I arpi ex- 

 pecting these specimens in a case which was packed before I left 

 South Africa. In England this species is abundant, though extinct 

 in many of our brick -clays, associated with remains of the 

 Elephant, Ehinoceros, Hippojootamus, etc. ; and in South Africa 

 the same genera of animals still exist with this little shell, although 

 during the last twenty years the larger animals are driven further 

 up the country, and but seldom appear in these haunts. About the time, 

 however, that I was in Hope Town two Hippopotami were reported 

 in the Great Orange Eiver near Hope Town, and many persons went 

 out after them, but with no success, although I believe some shots 

 were fired, but the tracks of the animals were visible, and were said 

 to be those of an adult and young animal. — James E. Gkegoky. 



PETROLOGY AND LITHOLOGT. 



Sir. — In the January number of the Quarterly Journal of Science, 

 the reviewer of the progress of Mineralogy during the last quarter 

 says, while noticing new works on Petrology — " Probably it would 

 be difficult to point to any branch of natural science which at the 

 present time occupies a more unsatisfactory position in this country 

 than that science which, according as it is pursued in the field or in 

 the cabinet has been variously designated Petrology orLithology, in 

 other words, tJie study of rocks, as distinguisJied from that of minerals. 

 No one can gainsay the first part of this quotation, as without doubt 

 books in the English language on both Lithology and Petrology, 

 especially the latter, are sadly required, the only work at all ap- 

 proaching to the latter science being Lawrence's translation of Cotta, 

 and any one who has studied it, must see how little the true science 

 of Petrology has been regarded in the compilation of that book. But 

 to return to the quotation — the latter part (now printed in italics) 

 seems to be highly objectional, as in its present form it can scarcely 

 fail to mislead students into imagining that Petrology is simply 

 the study of rocks in any form, while Lithology is the study of 

 minerals ; when in reality the former is confined to the study of 

 roclcs in mass, and the latter to pieces of rocTc ; by which means a rock 

 may lithologically belong to one class, and petrologically to another. 

 As for instance many of Cotta's quartziferous porphyries are litho- 

 logically granites, as they contain quartz, felspar and mica, while 

 petrologically they are Pelstones. A geologist divides rocks petro- 

 logically or into their natural divisions, and a mineralogist lithologi- 

 cally, as they Mdsh to make a multiplicity of " distinct varieties." 

 The difference between Petrology and Lithology has been fully ex- 



