IXXX PEOCEEDIKGS OF THE GEOLOGICAL SOCIETY. 



2000 metres, and this formation was not again submerged until about 

 the commencement of the Cretaceous period ; but before the close 

 of this period both these formations were again raised above the sur- 

 face of the sea, and there is not the slightest evidence that they were 

 ever again submerged, as the Tertiary deposits which rest upon the 

 Cretaceous have been entirely, or at least for the most part, formed 

 at the bottom of a great freshwater lake. The Quaternary or dilu- 

 vial beds were also formed in fresh water, and at a period when 

 this lake had been partially filled up and drained. 



The series of formations is therefore far from complete in the 

 province. Of the Palaeozoic period, the Devonian, Carboniferous, and 

 Permian are wanting ; of the Mesozoic or Secondary period there 

 are no traces of the Triassic, Jurassic, and a portion of the Cretaceous 

 beds; and of the Tertiary period only the Miocene occurs, there 

 being no certain evidence either of Eocene or Pliocene. The result 

 is, that the formations which are found in the province of Madrid 

 are Plutonic, consisting exclusively of granitic and gneissic rocks, 

 Silurian, a portion of the Cretaceous, part of the Tertiary, the Qua- 

 ternary, and recent formations. 



The author then describes these different rocks, commencing with 

 the granite, which occurs principally in three distinct masses, lying 

 in a N.E. and S.W. direction. But there are also numerous gra- 

 nitic islets in the gneissic rocks, many of which are so small that it 

 has been impossible to lay them down on a geological map ; they 

 extend into the provinces of Toledo, Segovia, and Avila, and even 

 into those of Salamanca and Caceres, forming one of the most exten- 

 sive granitic regions of the peninsula. He then describes in great 

 detail all the different varieties of granite, passing into kaoline, syen- 

 ite, pegmatite, micaceous porphyry, and many others ; also the ex- 

 ternal forms and structure of these masses, many of which are very 

 remarkable, caused chiefly by the unequal decomposition of the rock. 

 Isolated peaks and pillars, from 12 to 26 metres in height, have been- 

 thus produced, and the "^ mountain of the seven peaks " has been 

 crowned with its seven isolated craggy rocks. These granite masses 

 are often traversed by fissures of various width, but sometimes large 

 enough to be used as threshing-floors by the peasants. 



The author then enters into long details respecting the origin and 

 decomposition of the granite. He adopts the modern views that 

 water was no less necessary to its formation than fire, and thinks 

 that it must have formed the first crust of the earth, though he is 

 hardly prepared to say whether the granite of Madrid belongs to this 

 primordial granite, or to those masses which have been subsequently 

 erupted ; but there is no doubt that much of it belongs to the erupted 

 class. Some of the phenomena accompanying this process show that 

 not only have great disturbances and dislocations taken place, but 

 that the more recent and easily decomposed granites contain blocks of 

 an. older period, and of a harder and more compact nature. 



The gneiss is less extensively developed than the granite, but oc- 

 curs under a great variety of forms, as micaceous, felspathic, quartz- 

 ose, amphibolic, &c., all of which are described by the author ; it 



