1850.] RUXTON ON THE VOLCANIC ROCKS OF N. MEXICO. 251 



be recognized as far off as Bridport on tlie Dorset coast. At Leck- 

 hampton the Pisolite rests on a few feet of ferruginous oolite and 

 sand. The total thickness of this portion of the series is 42 feet. 



(8.) Immediately below this sand we find in the gravel-pits on the 

 side of the hill, an abrupt transition to beds of bluish clay. This is 

 the uppermost portion of the upper lias, the thickness of which can- 

 not be easily measured for want of sections, but which may be esti- 

 mated at 180 feet. 



(9.) Marlstone. This stratum, so rich in fossils at Bredon and 

 Alderton Hills, is not exposed to view on Leckhampton Hill, but is 

 probably indicated by the low ridge which rises to the south of Leck- 

 hampton Church. Its thickness may be estimated at 50 feet. 



(10.) Lower lias. It would be a matter of great difficulty to de- 

 termine accurately the thickness of this formation, from the base of 

 the marlstone to the top of the new red marls. Its depth is undoubt- 

 edly very great, and it is divided into several distinct zones, well- 

 marked by peculiar fossils. From an examination of numerous sec- 

 tions made during many years, I am disposed to estimate its total 

 thickness at probably between 500 and 600 feet. 



By adopting Leckhampton Hill as our standard of comparison with 

 other places where the inferior oolite is exposed, we may trace (as Mr. 

 Brodie has already done) the enlargement of some strata, the thinning 

 out of others, and the introduction of new ones. We shall find also 

 considerable variations in the organic contents of the same stratum at 

 different localities, whereby we may be enabled to study the causes 

 which operated in the distribution of animal life in the ancient seas. 



3. On the Volcanic Rocks of Northern Mexico. 

 By Lieut. G. F. Ruxton. 



[Extract of a letter to Prof. C. G. B. Daubeny, M.D., F.R.S., G.S.] 



I need scarcely apologize for drawing your attention to the fact, 

 which I am enabled to state from personal observation, that along 

 the whole ridge of the Sierra Madre and the continuous system of 

 table-lands lying along that ridge which may be said to connect the 

 two great chains of the Andes and Rocky Mountains, there are very 

 many tracts which have been subjected to volcanic agency ; and from 

 Durango to the point where the Sierra Madre joins the main chain of 

 the Rocky Mountains, on the ridge itself and the plains contiguous 

 to its eastern base, the tabular formation of the bluffs and hills, 

 called Mesas (tables) by the Mexicans, is a characteristic feature of 

 the country ; such Mesas being formed of basaltic lava and scoriae. 



At a point nearly 800 miles north of Durango, the Rio Bravo del 

 Norte forces its way for upwards of fifty miles through a volcanic 

 plain, and in the most north-western portion of New Mexico (Proper), 

 but still on the eastern side of the Sierra Nevada, volcanic districts 

 are frequently met with. 



The regions north of Durango having been rarely, if ever, visited 

 by Europeans, the information concerning them must necessarily be 

 exceedingly meagre. 



