Geological Socielij. 45.3 



sition limestone, grauwacke, grauwacke slate. All these, in the cen- 

 tral and southern parts of the county, are traversed by, or alternate 

 with trap rocks which are of various kinds, some being syenitic, 

 others hornblendic and amygdaloidal, whilst near Fishguard they 

 are columnar and basaltic. The beds of the stratified deposits are 

 frequently contorted, and their nature altered in contact with the 

 intrusive rocks. The transition limestone contains trilobites. 



The first of two letters addressed to R. I. Murchison, Esq., Sec. 

 G.S. F.R.S. &c. " On the Lacustrine Basins of Baza and Alhama in 

 the province of Granada, and similar deposits in other parts of 

 Spain," by Col. Charles Silvertop, F G.S., was then read. 



The Sierra Nevada, rising to the height of 1 1 ,000 and 12,000 feet 

 above the sea, is the culminating point of a number of subordinate 

 mountain groups which form a lofty chain stretching from Anda- 

 lusia on the W.S.W. to Murcia on the E.N.E. and bisecting in its 

 range the kingdom of Granada. 



This chain is composed of a central axis of gneiss and mica schist, 

 with successively overlying zones on each flank of transition and 

 secondary rocks, which on the south and along the shores of the 

 Mediterranean are, here and there, covered with patches of tertiary 

 marine deposits containing Sub-Apennine shells; whilst on the 

 northern flank of the chain, or towards the interior of Spain, the 

 secondary rocks are succeeded by formations of lacustrine origin, 

 which in the kingdom of Granada occupy two large and separate 

 basins, one near Baza, the other near Alhama. These great and 

 elevated depressions in the secondary rocks, though at little dis- 

 tances from the Mediterranean, are so cut off from that sea by the 

 Sierra Nevada, that their drainage is effected in a north-westerly 

 direction, into the Guadalquivir, and thence into the more distant 

 Atlantic. The author describes in detail the basin of Baza, which, 

 traversed by an insignificant stream called the Rio Baza, is sur- 

 rounded upon three of its sides by a secondary nummulite-Umestone ; 

 the precise age of whidi he does not pretend to determine, although 

 he states that it very much resembles certain varieties of the younger 

 Alpine limestone. 



Unconformably deposited on this and other older rocks, within 

 a district the average diameter of which is about thirty-five 

 miles, there are spread out formations of considerable thick- 

 ness, the organic remains of which are exclusively lacustrine 

 and tertiary. These in the immediate neighbourhood of Baza are 

 divided into two principal groups ; the lowest, consisting of marls 

 with laminated gypsum, sulphur and brine springs, is zoologically 

 distinguished by the presence of cypris ; the uppermost is a com- 

 pact, cream-coloured limestone, charged with many small Paludinse 

 of a species identical with one which is found in the lacustrine for- 

 mations of Central France. The united thickness of these fresh- 

 wafer groups in the neighbourhood of Baza cannot be estimated at 

 less than 300 and 40U feet; they are generally horizontal, but the 

 face of the country everywhere exhibits striking proofs of immense 



degradation, 



