IxXXiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



origin, and they appear to belong to the Miocene period ; conse- 

 quently there was here a vast freshwater lake, extending from the 

 Sierra de Guadarama to the Sierra Morena and Montes de Toledo, 

 and even to the mountains of Valencia and Murcia, although in this 

 direction it is somewhat difficult to fix its limits. The rocks of 

 which it consists are limestones of different varieties, clay, gypsum, 

 marls, sands and gravels, siliceous beds {pedernal flint), magnesite, 

 and conglomerate or pudding-stone ; the three former are the most 

 abundant, and they are not associated with any plutonic or volcanic 

 rock in the province. A considerable portion of the upper bed has 

 been removed by denudation ; by this means, and owing to the 

 violent action of the rivers during the Quaternary period, the physical 

 aspect of the country has been greatly modified. Low plains and 

 narrow valleys have been formed, which have been again partially 

 covered up by diluvium. In other places where the beds, which are 

 generally horizontal, have not been afiected by denudation, they 

 form vast upland plains, called wildernesses or deserts {paramos) ; 

 but where the upper bed of limestone has been removed by denuda- 

 tion the surface is much more irregular. Generally speaking, the 

 structure or arrangement of this formation is simple enough where 

 the strata are horizontal ; but this is not generally the case, as the 

 author proceeds to show in detail, and particularly in some places 

 where the inclination of the strata is almost vertical. 



This formation may be separated into three divisions. Limestone 

 prevails in the upper almost exclusively ; the middle division, which 

 has the least thickness, consists chiefly of clay and gypsum, and the 

 lowest consists of sandstones and conglomerates. The author then 

 describes in great detail various localities where these different beds 

 are found alternating with each other, and varying even within 

 short distances, the chief peculiarity being that the gypsum always 

 occurs in the central beds. The best building-stone in the province 

 is quarried in the neighbourhood of Colmenar de Oreja ; the quarry- 

 men give a diiferent name to each band. Some are better than 

 others ; but all are rather apt to split, and great care is necessary 

 in using them for building-purposes to see that the blocks are placed 

 in their natural position. 



The siliceous deposits are the most irregular in the whole forma- 

 tion. Sometimes the silex forms large masses in the clay ; some- 

 times it occurs as small_ angular blocks. ]N"ear Yicalbaro rounded 

 blocks of the same substance are found, and in one spot large irre- 

 gular nodules or slabs of flint form in the clay an irregular bed, 

 which is extensively quarried and used in Madrid. 



In order to account for the great variations which occur in these 

 beds even within short distances, the author refers to a suggestion 

 thrown out by M. d'Archiac, namely, that the material for these rocks 

 was supplied from copious mineral springs, and is not alone the result 

 of matter transported by the streams from the surrounding hills. 

 But this explanation does not appear to him sufficient, even without 

 taking into account those rocks which could not have been formed 

 in this manner ; and he observes that this variation is the more re- 



