AZUCAR TO THE COAST 109 



times is metamorphosed into various grades of 

 marble and in some localities well-preserved Am- 

 monites are found. In the highly altered and dis- 

 turbed sandstones and the more compact shales of 

 the lomas no fossils have as yet been discovered. 

 On the outer edges of the sandstones — to the north 

 and south — are occasionally found much denuded 

 patches of overlying Tertiary limestone. In the 

 deeper valleys among the lomas evidences of 

 an underlying serpentine may be detected, — the 

 basic foundation of the island upon which all the 

 sedimentaries have been laid. 



If the sandstones of the lomas can be accepted as 

 belonging to a later horizon than the limestone of 

 the sierras, a simple solution of the geologic struc- 

 ture of the region is at once suggested. Taking 

 the limestones to be of secondary age — probably 

 Jurassic — as indicated by the Ammonites, we may 

 imagine a horizontal strata of it of considerable 

 thickness resting upon the serpentine core of the 

 island and overlaid in the Cretaceous by the sands 

 and clays of the present lomas. Before, or at the 

 beginning of the Tertiary, a warping or folding of 

 the strata along the axis of the Organos Mountains 

 produced the wavelike conditions of strata shown 



